ANSWER: When you register an account, it is not immediately active. You will recieve an email within 24 hours of registering. This email will contain a link that you must click to activate your account. Once your account is activated, you'll be able to log in. If you do not recieve this email within 24 hours, contact us at email@example.com.
If your account is already active and you're still having trouble logging in, you may be mispelling your username and/or password. On the login screen there is a link to have your username and password emailed to you. Try that and make sure you are entering them exactly as they appear in the email we send.