You can login here using the GATE Overflow username (in lower case) and password would have been sent to your registered email address (this password is different from your GO one). If you lost the email, you can do Forgot password option. For new users, the email should arrive within some minutes of registration (Registration can be done here)
Those who are logged in can click the Site Home, then the available courses and Enroll on them.
Please see if you have any trouble.