The Fix Facebook Like plugin fixes problems like wrong thumbnail, title, description, URL, and gives few more options for your WordPress.
No, its optional, it will still work fine.
You need your numerical Facebook ID.
To get your profile ID, click on your Facebook profile and look at the URL, it should resemble this: http://www.facebook.com/profile.php?id=123456789 where 123456789 is your Facebook user ID.
If you have a username, lookup your user ID with this URL: http://graph.facebook.com/username.
Visit Fix Facebook Like Plugin Homepage for more information.