As a student, instructor, or staff member at a school with a Higher Education subscription, your school itself is the subscriber, and there’s nothing further you need to do to sign up! As long as you're accessing newplayexchange.org while on your campus network (whether that's physically on campus or off-site using your school's proxy IP address), you should see a welcome message for your school at www.newplayexchange.org. You don't need to log in from there, though if you also have an individual subscription, you can.
With a Higher Education subscription, you can search for, read, and download scripts, search for writers, and contact them (or their representatives) directly.