If this is a new account, you won't be able to log in until you've confirmed your email address. Right after you registered, you should have received an email at the address that you registered with us. This email contains a special link. Click on the link to prove that you can receive email from us. If you did not receive the email, make sure that you did not misspell your email address when you registered with us. Check your junk mail folder to see if the email message wound up there.
If this is an old account, check that you are entering the correct username and password. If you've forgotten your password, you can recover it.
In either case, make sure that cookies are enabled for your browser. If all else fails, report the problem in our support area.