You need a Celtx Account to log into Project Central, which is a different account from that of the Celtx Forum. You can register for a Celtx Account at Celtx Studios.
You can reset the password for your Celtx Account via the Password Reset form at Celtx Studios.
Note: If you are experiencing trouble logging in with your username and password, ensure you have cookies enabled in your browser, then try logging in again. Cookies are required for the authentication process.