ORIGINAL: dedmonwakin
That should go on top of your airsoft gun or actually the underside of the turret where it rests between the turret and airsoft unit. You should see a circular bracket where it would rest. This pushes your airsoft gun down keeping the barrel facing upward. The elevation unit will push the airsoft housing up by that tab at the end, compressing the spring as a result forcing the barrel down.
Also, to ease reassembly, you should superglue the spring to the turret roof. That holds it in place while you manipulate the rest of the turret.
Jim