The ranger's office is almost always closed at the moment (Funding?). Big rock is signposted and is found by following the first left after the Rangers office, then the first right (staying on the sealed road). Or if you take the first left and follow the circle drive for about 3-4 k you will pass the closed off road (on your right) that will lead you to Urinal walls and the saddle areas before you drive right under Royalty walls, also on your right. After this you will pass the well signposted carpark for Gravel Pit Tor on your left (btw the crag is now up behind you on your Right), then the Mountain Bike track starting point on your left before heading down the hill past Cressy Gully road (gate on your left) and into the open area where Nightfall Pinnacle is up on your right. Then its on past In Broad Daylight Chasm, Corio Ridge Tor and back round to the sealed road where you end up where you started. Or can also drive up to the Turntable carpark and access all the crags below (like Flinders Peak slabs) and above the circuit walk (like the overlap).
Probably worth exploring the more obvious and accessable areas a little first and then go get the guide :)