Have you really registered?
Seriously, you must register in order to log in, in order to post or reply to messages.
After you submit a Username, Password, and valid email address, you'll automatically receive an email asking you to "Activate" your account. (make sure it isn't just sitting in your spam filter or junk mail folder)
You must click on the Link provided in that Email, to activate your account to post on the message board.
If you've registered and you still can't log in, then check and double-check your username and password. Watch out for the "Caps Lock" on your keyboard. Make sure the upper or lowercase letters are just as you submitted it.
Usually this is the problem; if not, contact the board administrator -they can reset the username or manually activate the account. (please allow one full business day for this)