How do I change the Donate button to "Add to Cart" and link it to my pay pal account?

You can't using the built in Donate Module. The built in module is just a simple Donate button that links off to your PayPal account. There is, however, all kinds of neat things you can do at PayPal.com in terms of setting up mini stores, buttons, etc. After you make whatever you want they give you a snippet of copy and paste code you can just drop into your site. Thus if the simple Donate feature is not doing exactly what you need, I'd encourage you to explore the options available directly at PayPal that you can then plug into your class web site.

Where does one find this "Donate" button that is built in and how do I activate it?

http://forums.classcreator.com/messages.cfm?threadid=F9E0A442-F738-CCCA-C4023CF6543A3981