My guess is that you really want a non-stop flight, not merely direct. (Direct flights can have a stop along the way.)
For that, you're best bet is to fly out of Detroit Metro on either of Delta or Spirit. Personally, I hate Spirit.
Southwest also flies out of DTW, but they haven't done non-stop flights between the two airports in a few years. There are a number of one-stop flights where there can be a plane change.
Airtran flies out of both DTW and Flint. There used to be non-stop flights, but those are no more, too (I'm pretty sure).
Finally, if you'd consider flying out of Lansing, SunCountry has a VERY LIMITED schedule of non-stop flights between Lansing and MCO. The schedule is too limited to me and I would not be willing to chance their not having an issue where it could be a day or more before a flight can get you where you want to be.
United and US Air also fly out of DTW and LAN. American is another airline that serves DTW, but with their pilot issues right now, I think booking with them would be foolish.
Try doing searches on Orbits, Expedia, Kayak and the like, in addition to Southwest, so you can do some comparisons. When you find a fare I like, I go directly to the airline to book. That way, if you need to make a change or something, you don't have to deal with the third party.