If this is a new account, you won't be able to log in until you've confirmed your e-mail address. Right after you registered, you should have received an e-mail at the address that you registered with us. This e-mail contains a special link. Click on the link to prove that you can receive e-mail from us. If you did not receive the e-mail, make sure that you did not misspell your e-mail address when you registered with us. Check your junk mail folder to see if the e-mail 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.