First of all, you need to actually get the belt. In order to get the belt, you need to give Ranger Rick the elk antlers he asked you to find. Ranger Rick will give you a junior ranger badge that you will show two scientists to get the magnetic belt.
In order to move a rock, use the belt then click the rock and drag the rock. In order to be pulled towards a rock, use the belt then click and hold the rock. You may have to jump to be able to click the rock. The creators made a video showing the magnetic belt in action.