most parks and wma's close at sunset and are illegal to enter at night without permission. so it's entirely your risk and you would be commiting a crime(tresspassing), buuuuut, i really don't think you'll be bothered. noone is patroling the trails at night. as long as you stay away from campsites, ranger stations, park headquarters and parking lots, you should be ok. so basically, you would either be riding there from your house, or parking at the nearest supermarket/strip mall/shopping center and riding in. at some parks, there are parking areas, both official and unofficial, that are not locked at night. allamuchy and mahlon at least. if you're not out too late, it shouldn't be a problem. but if you plan on being out there till ten or later, best not to park at the park.
your best bet is to park while the sun is low, but still hasn't set. that way you get warmed up and used to the trail, and you can gradually adjust to riding at night. doing it that way also gives you a valid excuse should a ranger or park cop confront you about being int he park after hours. just tell em you got lost.
NEVER NIGHT RIDE ALONE!!!