Google should be able to get you there:
http://maps.google.com/maps?daddr=44.99719,-123.05808+%28Keizer+Rapids+Park%29
They recommend going through Salem, though, not the ferry. Apparently it's farther but faster.
Once you're in Keizer, it's easy: Chemawa Road is the main east-west thoroughfare. Follow it to its western end and then turn left as soon as the road turns to gravel and you enter the trees.