As you are looking at the Valley Green Inn, make a left down Forbidden Drive. There will be a switch-back left hand turn a while down the trail. It is easy to find because it will be the only trail on the lefthand side. It crosses a bridge (wood and cement). You will end up at the bottom of a hill. You can go up the hill, go left or you can go up another hill to the right.
How to get there:
From Center City Philadelphia: Head down the East River Drive and then go up Ridge Avenue. Follow Ridge Ave until you hit Roxborough Ave. Make a right on Roxborough and get on Henry Ave. Follow Henry until you see a sign for Valley Green. Make a right and follow the forest road until you get to the parking lot.