it is behind the wire mesh that supports the foam filter element. there should be a rubber gasket around the mesh, so it takes a tiny effort to get it off, but you arent going to fuck anything up.
anything else you need to know about that bike? i work on them, and own an 87(?). solid little bike, the only thing about them is the rearend. make sure the two nuts that hold the axle tight, are tight. and check the oil level here and there.