Okay I've fixed the problem and I think that I should return as a matter of courtesy to anyone else who encounters it to post the solution. As it turns out, I avoided the nesting problem with the file structure -- but thank you anyway mikey1, I heard a couple of nice tips when I watched those videos so it wasn't time wasted and I appreciated the link.
SOME PRELIMINARY NOTES:
I had two problems with my FTP client, FileZilla:
1. I needed to upload the plug-in files in ASCII file transfer mode, rather than "Auto" which I used initially. This probably resulted in uploads proceeding in binary mode, which I noticed in a discussion of phpmyadmin problems on another page up on this forum was not recommended for php files. That was on me.
2. For some reason I cannot explain, both before and after setting the file transfer mode to ASCII, FileZilla would become disconnected from the server during the upload. I am using broadband dsl, I am certain I have the latest version of FileZilla because I updated it a couple of weeks back, and I have had nothing but success with FileZilla ever since I began using it over two years ago. I am at a loss to explain why this happened, but I went and downloaded FireFTP for Firefox and used it instead. On my fourth upload attempt (1st with FireFTP) it did the job perfectly, with no disconnects and the plug-in activated from the admin backend on the first try with full functionality.
I should mention that earlier I also was unable to configure one of the phpmyadmin plug-in versions I downloaded from SourceForge.net, even though I transferred it up to the server in ASCII mode, but I cannot say whether this may have been due to the disconnect/reconnect problem with FileZilla or not, because I notice that its folder structure is "universally named" rather than specifically named to WordPress (phpMyAdmin vs. wp-phpmyadmin for parent folder name). FileZilla reported that all files transferred successfully (0 failures). So I also recommend using the phpmyadmin version linked from the "Plug-ins Directory" page here at WordPress.org, which is the one that worked for me.
So, given that I am an experienced developer (I'm a .NET guy) who has just had the rare experience of going through fits to solve a problem, I am including the following instructions for anyone else who may have trouble downloading, installing, and activating the phpmyadmin plug-in for use with WordPress. I do not present them as a generalized guide, but rather as the specific details of what worked for me.
********* INSTRUCTIONS FOR INSTALLATION OF phpmyadmin PLUG-IN *********
For WordPress Version 2.5 Installed on Apache (version unknown)
Server File Path to Plug-ins folder: .../www/wp-content/plugins/
BEFORE YOU BEGIN
<li style="display-style:disc">Download or upgrade to latest version of Firefox browser. (http://www.mozilla.com/en-US/firefox/)
<li style="display-style:disc">Get the latest version of the FireFTP ftp client for Firefox and install it. (http://fireftp.mozdev.org/)
<li style="display-style:disc">Create a folder on your local PC to hold the download of the zip file for the phpmyadmin plug-in.
Make sure you have zip software installed on your PC.
1. Go to the WordPress phpmyadmin page at http://wordpress.org/extend/plugins/wp-phpmyadmin/ and click the "Download" button at the right of the page. Version as of today, August 15, 2008, is 2.10.3.
2. When prompted for the download, click "Save" and make sure the target folder on your local PC is the folder you created to hold the download (See "Before you begin" above), which is a zip file.
3. After the download completes, navigate to your local folder and unzip the download, wp-phpmyadmin.zip, and extract the contents to the same local folder within which you have saved the zip file.
4. When you view the extracted files you will see a parent folder named "wp-phpmyadmin" with a child folder of the same name "wp-phpmyadmin". It is important to note that the parent folder is only a container for the plug-in. The folder you will care about is the child folder "wp-phpmyadmin" when you upload.
5. Open Firefox and go to the "Tools" menu and click "FireFTP". You will be presented with the FTP client interface.
6. To configure your FTP connection, type in the ftp address to your server, along with your username and password and click "Connect".
7. When you have connected to the server, go to the bottom of the FTP client interface, just above the browser status bar, and locate the File Transfer Mode setting. The options in FireFTP are "Auto", "Binary", and "ASCII".
8. Make sure the selected mode is "ASCII". You can change it by a left-mouse click.
9. In the leftmost pane of the FTP client interface navigate to the local folder where you saved your download and which now contains the extracted files.
10. TO AVOID NESTING, GET THE FOCUS TO THE wp-phpmyadmin CHILD FOLDER ON YOUR LOCAL PC: You must get the focus to the parent folder in the leftmost pane to enable the local focus in the second pane from the left on the child folder you wish FireFTP to upload. By way of explanation, if the folder you created to hold the download is entitled "MyWordPressSite" and it is on your "C" drive then, in the drop-down list box that spans above the two leftmost panes, you should see the filepath "C:\MyWordPressSite\wp-phpmyadmin" listed (that is the parent folder file path and that folder should be shown highlighted in the leftmost pane) and in the "focus" pane, which is the second pane from the left, you should see a folder icon directly underneath "Name" that is named "wp-phpmyadmin", which is the child folder.
11. GET THE FOCUS TO THE TARGET PLUG-INS FOLDER ON THE SERVER: In the third pane from the left (second pane from the right) open up folder "www" and then underneath open up "wp-content" and then underneath open up "plugins". In the drop-down list box that spans above the two right-most panes you should see the file path: "/www/wp-content/plugins".
12. Return to the second-pane from the left, with its focus on the child folder (check the file path in the drop-down list box as shown in No. 10 above) and left-click once on the "wp-phpmyadmin" folder to make sure the focus is set.
13. Click the right-pointing-arrow between the second and third panes in the middle of the FTP interface to upload the plug-in to the server.
14. When the upload is finished, check the file path on the server-side. If everything is done correctly you should be able to navigate to the uploaded folder in the third pane from the left (second pane from the right) and see the file path "/www/wp-content/plugins/wp-phpmyadmin" shown in the drop-down list box spanning the two server-side panes and in the rightmost pane the folder shown (with a couple of files underneath) should be "phpmyadmin".
15. You can close the Firefox pane and open another one and go to your admin interface in WordPress. Go to "Plug-ins" and find the "wp-phpmyadmin" plug-in listed and click "Activate" and the plug-in will be available for use if your WordPress installation is sound. You can find the "phpmyadmin" link under "Manage" in the admin interface thereafter.
********* END OF INSTALLATION INSTRUCTIONS FOR phpmyadmin *********
I may have to return to this forum for advice on securing and/or working with additional plug-ins in the near future, so I hope I may have earned some points by posting these precise instructions.