The HomeLink system works with two types of door systems. The ones that have a locked frequency (older garage door openers) or the new style that have a rolling code...meaning the code changes everytime you open the garage.
Either way, that information can be found on the HomeLink website. It's basically a procedure where you need one of your handheld garage door remotes, access to the "learn" button on the garage door opener itself and your van's buttons.
GO to this website and it should answer your questions, if not let me know and I'll try to explain in more detail.
http://www.homelink.com/program/gdo.html
Jeff
P.S. Although the photos don't match the van's buttons, the HomeLink system works the same in all vehichles. My GMC Jimmy and Chrysler T&C programmed exactly the same way.