If you have logged in before, then just start the game via the hd mod shortcut, that opens the launcher and you just start the game normally. Then go to the online section in game, and use your server account name for the username, your server password and a nickname of your choice. You can login to t3aonline.net with your forum account to see/create a server account if you haven't done so already.
Ignoring the HD mod in this thread.You say enter your server account name and password, and it should work. I do that in, BFME2, and it gives me this message: "The EA registration application can not be found in the install directory. Registration is required for online play."
It also gives me this when I press register with the incorrect credentials, or no credentials at all. I did download the T3A launcher before I logged in, but I'm not sure what that would do. The game is hooked, and in version 1.09.