To be eligible to register to vote you must be:
You can register online or mail in a paper application up to 15 days before an election. For this year, that date is October 24. Mailed paper applications must be postmarked before October 24.
November 8, 8:00pm:
If it's less than 15 days before the election, it is not too late to register! You can register to vote all the way up until 8pm on election day itself by going to your city or township clerk's office in person.
Michigan Elections Director Jonathan Brater gives an overview of voter registration in Michigan.