All ATVs in Wisconsin must be registered, even if they're going to be used exclusively on private property. To register your ATV, visit the Go Wild website, visit a sales agent, a local DNR office, or send in your application by mail. If you register your ATV for public use, you must display two annual registration stickers, on either side of the vehicle, and a rear license plate.
Check out these Racine, WI ATV/OHV resources: