As a Plus member, you should have a "Plus Member #". That's your entry key to Plus-only resources throughout the site.
Anytime you click on a resource marked with the + icon, you'll be asked for the code. (Actually, if you use the code once, the system should recognize you (and allow you to skip the code step) for a while.
OK? Any questions or problems, call our Plus hotline. They'll walk you right through it.