Cache itself is not on or by the arch. It is placed up the was a short distance off of Keyhole Flat where the wash emerges from the canyon.
Cache is under a pile of rocks at the base of a tree it should be easy to see. It is at the base of the hill, not up the hill.
Getting good coordinates was difficult for some reason. Blame the trees, blame the rocks, blame the canyon, I don't know, but I avereaged a few waypoints so it should be fairly close. If it seems off take a waypoint and post it in your log and I can average that in to my others or use it instead.
To view the arch itself you will need to drop down in the wash of Dry Canyon and go to the West. This is a great arch that I have not been aware of.