Take exit 25 for WA-18 W off I-90 E, after ~4 miles you should see the sign for Tiger Mountain State Park on your right.
Alternately, “Tiger Mountain, WA-18, Issaquah, WA 98027” in Google maps should get you to the parking lot.
Reconnoitered the trail last weekend. There is an unpaved road to the GPS coordinates listed at the WTA org link, but the gate was closed for cars, so we’ll be hiking up that road to a view of Mt. Rainier. Lots of salmonberries as we get to the viewpoint (I didn’t eat them all).