Adding an expiration date and registration code to Protected Content pages

Hello WPMU community,

I've developed digital tools to go along with the Biology Lab Manual I've written. I'm trying to protect the content on the site, so they can only see it if they bought the lab manual (there is a code in each manual). Is there a way to modify the registration page so they have to provide the code to complete their registration?

The site's current registration page lists the student membership as "free." It's not free - they have to have a code. Can I change the word "free?"

I also want the membership to expire in 6 months. How do I add an expiration date?

I'd also like to further test the page, but I'm unsure of how to do that, since I already show up as admin. I've tried the tools, can't quite get it.

Thank you,