Good question. I grew up in the woods of central MS, so having a place to ride was never a problem when I was a kid. But unless you own your own woods or have a mutual agreement with a neighbor with plenty of open woods, or belong to a hunting camp and use the ATV like a lot of hunters do, I don't know of many, if any, public ATV riding areas. Most of the parks I've been to don't allow them, but that would be the first place I'd look. Check out the MS State Park system.
I'm looking at a few sites right now, try Trace State Park I think in the northern part of the state.
Also, here's a quick short list:
http://www.visitmississippi.org/outd...Group=Consumer
*If the link takes you back to the main Outdoor Recreation page like it's doing to me, just click on Trails (over on the left) Then select Off Road Vehicles/ATV Trails.*