Grab the cube to your left and set it to roll down the stairs and land on the switch. Head right through the doorways to the upper level. From there, peak around the corner and grab the energy ball out of the socket. The socket is opposite the entrance stairway and on the mid level. Energize the doorway socket at the top of the collapsing stairs, hop down and walk through. Alternatively, you can take the cube with your and throw it off from that top level to land on the switch.