There are two types of attachments available from Vapor Trail; one is a sort of "button" that slips between the forks in the upper limb @ the cam, the second is two aluminum (?) bars about 1/4" square with a machine screw which attaches the two pieces together (put one on top of the limb, the other goes opposite it, and the screw clamps them to the limb) also at the cam. There's a small hole drilled in the bottom piece.

Or, as an alternative, guys ignore the clamp and just use D-loop thread (and a D-loop knot) to tie off to the side of the limb (right hand side for right hand bows).