The entrance most used is found on Mapleton Forest Drive at the Mapleton Day Use Area about 3.5 k's from Mapleton itself. Drive up the range to the Mapleton Pub and then turn right and then right again into Mapleton Forest Rd. Follow the signs to the State Forest. At around the 3.5 km from town mark you'll find a picnic area, start there.
This area has several options of fireroads to explore including the run out to the lookout. Some of the tracks are quite narrow and challenging as well. Be aware it is a legal and popular trailbike region so best to give way early to them as they don't expect many mtb riders there, especially further into the forest.
One cool little ride on the east side of Mapleton Forest Rd down to Kureelpa Falls. It provides a nice, long climb back out after enjoying the falls themselves. Take care around the falls as you'll be at the top of them.
Another good ride out to the lookout back on the west side of Mapleton Forest Rd. Great views and cool spot to refuel before heading back.
There are plenty of other fun fireroads to explore. Can be boggy in and after wet weather.