Create a Backside Login Page - Step IV
37. Click on the (+) sign in the Server Behaviors Panel (CTRL + F9) and select User Authentication > Log In User and use the following settings in the Log In User window.

38. Click OK and save the login.asp page.
That's all there is to creating a Login Form in MX.
By the way, try using Admin as the Username, and admin as the Password.
Click here to get an example page.