Hello!
If you are unable to log-in, make sure that:
-Your username is not more than 16 characters. If your in-game username is also xxXMegaMonsterXxx, I'm sorry, but you'll have to make a new account. Luckily, registration is still open if so, so you could hurry and make a new account right now.
-You are using the numbers above your keyboard, not the numpad.
-Caps lock is off.
-Your username doesn't have odd symbols.
Hope this helps! ^^