If you're coming from the East it's about a mile before the road to Timberline on the left. There is a sign that says "Trillium Lake". make a left there and go about a mile. the first right turn into a gravel road is the Airstrip. If you keep going you'll be at the Pay lots... F them. You'll be close enough to Trillium to bathe and play in the lake.
If you're coming from Portland pass up the Road to T-Line and just look for the trillium lake sign. it creeps up on ya.