After making the payment, you will receive unique NMAT by GMAC Registration ID through email. Upon logging into the official website through the credentials, you will be able to schedule and download the NMAT by GMAC admit card.

Because now the registration dates are over, you will have to check by logging in if you can still schedule.

Officially it states 'you may reschedule your appointment if you can't appear for the original test appointment. You may reschedule your appointment up to 72 hours prior to your original appointment. You may pay for your reschedule using credit card only'

Check here for the dates and charges.

Also go through the complete NMAT application process here .