From Uster : take the Winterthurerstrasse to Gutenswil, drive through Hard Wald. On the right you will see a quarry - take the road to the right before the quarry. The forest cabin is on the right.
From Gutenswil: take the Winterthurerstrasse to Uster. Just before you reach Hard Wald there is a quarry on your left, take the road after the quarry on your left. The forest cabin is on the right.