There are a few possible reasons you aren't able to log in.
If you are having trouble logging in, try resetting your password.
It is also possible you are entering the wrong username. If you can't remember your username, enter the email address you signed up with through the Forgot Username link.
If neither of these options has any effect, your account may not yet be active. Check your email for an activation message. Check junk/spam folders as well, just in case.
If you have received it, click Activate Your Account in the activation email.
If these options do not work, or you have not received your activation email please contact email@example.com.