On the thing where you make shapes like boxes, click Joint next to extrude and click where you want it on your model, then go to the 'Joints'. Click on the joint you just placed, it will be called joint1 or something, then.
For where you want the model held, type in 'mountPoint', and click rename.
For where you want it to shoot out of, if it's a gun, call it 'muzzlePoint'.
And to select a joint to move it around, click on Select, then on 'Select options', click Joint. Then click the joint you want to move, and use the move thing.