Here is a link to the Secretary of State where it's explained:
DNR - ORV Licenses: prices, requirements and how to buy
You cannot legally register an ATV to ride on public roads in Michigan. Some counties and municipalities have adopted ordinances that allow some riding under certain conditions. Unfortunately it varies from one area to another and unless you're a local it's a lot of researching and information to be gathered about the local laws...
More info on the subject:
DNR - Where to ride