Once you sign in to https://signon.stjohns.edu, you should see an app called Medicat. 
If you have not logged into the Sign-On Portal yet, please follow the instructions listed here:
Once you click on it, please follow the instructions in the link above.
If you are able to login to medicat, but have an issue with Medicat finding your account, contact Student Health Services: http://www.stjohns.edu/student-life/health-and-wellness/student-health-services

Health & Wellness (Student Health Center) contact info: 718-990-6360 (option #4) 

Hours of operation: Monday – Thursday 8:30 AM – 4:30 PM and Friday 8:30 AM – 3 PM EST 

*SJU offices are closed on Fridays during June and July.