Installing the Magento Facebook Connect Extension is easy whether this is your first Commative product or you're just adding on.
After purchasing your extension you'll receive an email confirming your order and giving you a link to your new Account Center where you can use the Download link to download your software. Save the .zip file locally on your computer.
Log in to your Magento Admin Panel and disable your cache by navigating to System > Cache Management, selecting all the cache types, and setting them to "disable".
Once you have the software, unzip it and you'll find one folder with all the important files you need. FTP the contents of that folder (which may contain sub-folders like "app") into the ROOT directory of your Magento installation. You know you're in the root directory of your Magento installation when you see folders like App, Design, Skin, etc.
Commative extensions will not overwrite any core Magento files because they're built the "Magento Way". The only time you should see overwritten files is if you've previously installed Commative extensions because some of those required files are duplicated across extensions.
*Be careful when copying these files over that you don't accidentally copy them in to one of your sub-folders. Before you grab the files you want to copy over, make sure you can see files, not folders, in the Magento directory you're going to copy to. If you drag your files and release the mouse over a folder, your files will end up in that folder. If you do it over other files, your files will go in the directory you want.
Once you've done all the above all you have to do is log back in to your Magento Admin Panel and re-enable your cache. You should now see your extension installed in both the front and back end.
Once you have the extension installed make sure you know how to configure it.