Install plug-in

Hi,

I’m trying to install a Editorial calendar plug-in and run into some troubles:

When I click on Plug-in in the Configure menu it says that I need to upgrade, even though I’m premium member already.

I’ve also been trying to upload the plug-ins to the wp-content/plugins/ folder instead and have read quite a few threads to understand how I should do this, but still don’t get it to work.
I’ve downloaded an FTP client, FileZilla, entered the address “ftp:// and then the blog name.com. Login and password I’ve chosen the same as for WordPress and port I’ve left blank. It doesn’t work.

Can someone explain in a simple language how I can install plug-ins?

Many thanks

Melia