Hii

Please visit the official site for login page. Here is the link - http://mbbs.aiimsexams.org/
There you will find Applicant login zone.
Fill in your registration number, RUC and password and then login. 
You will be directed to your account MyPage.
There you can proceed to step 4 and fill in the details. Next make the payment and choose your city for examination.
At last print the registration form and you are done.

Hope this helps
Thanks!!