You need to work through the Michigan's Charitable Gaming Division to get a license. This state agency governs the licensing of all gaming in your state - including charitable bingo and raffles. The web site is:
www.michigan.gov/cg
I only glanced at the requirements, but it looks pretty similar to our rules in Texas. The first time around, you fill out a fair amout of paperwork - including providing quite a bit of history and information on the organization. The paperwork needs to be submitted at least 30 days (Michigan - 4 weeks) ahead of time. You pay a small fee (Texas=$25, Michigan =$15). Then, if all goes well, you get your license.
The good news is that subsequent applications are much more streamlined. We're applying for our second year's license right now, and it's really very little paperwork compared to last year.
Bingo IS gaming and is big business, so it's pretty heavily regulated. I know this seems like some extra hoops to jump through, but I firmly believe the PTO needs to set a good example by following state law on these things.
Good luck!