MockupTiger online login is through the username you create during the signup process.
But we noticed through our logs that people try to login using their email address. Once they realize that email is not logging them in they try the username for login purpose. This is a time waster and bad experience for the user.
That is why most of the established websites allow you to login using either the username or the email address.
MockupTiger is going to fix that soon in the immediate release.
10-Aug-2011 - UPDATE: Users should be able to login using either of the username or the email id