First, check that you are entering the correct username and password. If you've forgotten your password, you can recover it.
If this is a new account, you will need to confirm your email address. Right after registering, you should have received an e-mail at the address that you provided. Click on the confirmation link in this email.
If you don't see the confirmation email, check your junk mail folder.
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If all else fails, report the problem in our support area.