erm, there's the front down by harrison drive, as you go to the car park by the derby pool and you see the big hill, just behind that. that'll probably be a good place to start if you're not experienced in offroad quadding
otherwise there's a dirt track in here (behind the trees that you can see): http://goo.gl/maps/ooue
(browse around the map to get an idea of the location)
*i'm no quad biker, they are from my experience of offroad mountain biking and they're places i sometimes go to just mess about, but they'll be decent for a quad too.