Here's the link to the MN DNR license page....
http://www.dnr.state.mn.us/licenses/fishing/index.html?type=fishing"Dark houses, fish houses, and portable shelters placed on the ice for shelter while fishing do not need a license unless the house will be left unattended overnight. See fishing regulations book for display requirements Fishing House or Dark House."-MN DNR 2011/12 Fishing Regulations Rulebook