You'll have to sign in at the visitor board in the parking lot. Maps are available. Trail system is only open sunrise to sunset, and there is no camping, fires, dogs, or fishing allowed.
Trail is mostly doubletrack with some singletrack mixed in. It's not very technically challenging, but there are some unmarked spurs that you can get lost on and have some fun. Follow the loop counter-clockwise for a steady uphill grade and a short (1 mile or so) downhill romp, or go clockwise for a steeper and more rigorous climb followed by a leisurely downhill.
From 654 in South Williamsport, turn south onto Jack's Hollow Road, go about 1/2 mile, then turn left onto Mosquito Valley Road. Continue over the one lane bridge, go all the way to the end of the road, and park at the Williamsport Water Authority visitor lot on the left. Trailhead starts on the dirt road next to the parking lot and is marked with a green diamond.
http://www.singletracks.com/item.php?c=1&i=3213