The red parts are what I added in. Basically you drive south from Kihei and head for a beach called Big Beach. Any one should be able to give you directions there, and I think there are signs. It will be after the road goes away from the coast line and you start getting into less civilized areas.
Big Beach is just a normal beach.
When you park at Big Beach, walk down to the beach and then turn right and follow a path that leads up into the cliffs to the north. This path will lead you over the cliffs and down to a secluded clothing optional beach.