Ok...more specifically you need to go here:
http://developers.facebook.com/
You must be logged in under your personal TL, NOT as your biz page (at least that is what I found); if you have not already done so, you need to register as a developer. Then you can set up an app to link to this plug-in - click on Apps in the menu and it will basically walk you thru it. (If you've already registered just go to Apps and select Create New App in upper right.)
To set up the App you need to give it a name, which is mainly for your reference; you do NOT need to fill out the Namespace field (although I did before I knew that; it doesn't seem to hurt anything). Go thru the Captcha stuff.
The next screen is a bit overwhelming but you only need to fill out one item for the purpose of this plug-in. Go to the section "Select How Your App Integrates with Facebook" and open up the "Website with Facebook Login" part (just click on the arrow). Put your website url in the field - be sure to include the http:// part. Save changes and you should be done on FB.
Copy & paste the App ID & Secret into the plug-in in WP and follow the remainder of the directions there.
Sorry to bore you guys that "know" all of this, but I didn't and have literally been trying to get this plug-in to work since Christmas, so just wanted to share this. The primary issue is that the instructions for the plug-in don't reflect the latest version of FB Developers area...hopefully that will be updated soon for others.
cheers