Park at the top of Tolosa st. near the big metal gates.
Push your bike through the gates and walk up the road, left past the 4X track and past the dirt jumps and you'll find the start of the access track. Follow this up the hill for about 50m until you reach a fire trail then continue along and take the next fire trail on your left. Then push up up up! About a 20 minute walk.

Zoom in on the map for a pretty exact location of the starting ramp.

The track is of national standard and recommended for advanced riders. That said, there are chick runs around all the hard stuff.