They have a site that talks about what villas are open for tour Between 9am-8pm. If you are staying at that resort, they say you can just Drop by. If not, you call the number on the site to setup a time and just let the gate guard know you are going for the tour. Discoverdvc.com is the site, scroll down to the open house section.
Also, even though the grand villa is not listed for Rivera, if you ask nicely they will probably take you up if it is open, which we got to do along with everyone else that was going that day.