From Seattle and the Eastside
Take I-90 eastbound to exit  #32 (436th St).
Turn left and take 436th back over I-90, until it dead ends at North Bend Way.
Turn left, then take the first right - Mt. Si Road. 
Stay on Mt Si Road for 3 miles (Look for the Rainbow Lodge sign on your left - exactly 1/4 mile past the Mt Si Trailhead).

Please note that Google Maps has an error which leaves you short of the lodge entrance by several blocks. 

Rainbow Lodge Retreat Center

PO Box 963
46820 SE Mt Si  Road
North Bend, WA 98045

425 888-4181  
 rainbowlodge@rainbowlodge.org