Dolly Sods and Cranberry Wilderness are both National Forest Wilderness Areas, and as such do not allow mountain biking.
I'd suggest looking at the longer bits of legal singletrack, and try linking them together with gravel roads. The Blackwater Canyon trails, North Fork Mountain, Spruce Knob down to Seneca Creek - all have some mileage. I'd say Allegheny Trail, but I hear that the maintenance is still very spotty.
Another thing I've noticed is that a number of backpacking trails that were open to mtb are no longer maintained. Not sure how widespread this is in MNF, but several in my old backpacking trail guidebook are not shown on the trail inventory.
You can check the WV trail inventory map to get some idea of what is available, though I'm not convinced that it is totally up to date as to open trails, and allowed usages.
http://www.mapwv.gov/trails/Good luck. IMO you're looking at one of the nicest areas in the eastern US to get out in the woods.
Steve Z