The stop pin should be installed in one of the scope mounts. That is the one you will use as the rear mount. It should be a threaded steel pin in the center of the mount. You install the mount loosely, position the mount approx. over the stop pin hole in the rail and then use the small, supplied allen wrench to screw the pin down into the hole, pushing the mount back so the pin butts up against the back of the hole, then tighten the mount up on the rail.
If you did not get mounts with a stop pin, I would exchange them for a set that has it. The Gamo 1000fps springers all recoil enough that they definitely need to use a stop pin.