The area is behind the fairgrounds in Shelton. It is private land and technically only open for events or for PSER club members. Go to the NMA website and they should have some information on PSER. Bottom line is it is only open for races and poker runs unless you are a clube member. If you don't have a course to follow, or a guide, it would be pretty tough anyway because there are probably 100 miles of trails, none of which are marked unless it is a sanctioned event. PSER has 4-5 events a year. Best way is to go to one.
If you want legal casual trail riding, Belfair (Tahuya) is probably a better bet year around.