Lay the cabinet back down on its side.  Attach the four casters at the four corners of the base. The two I put on the front had brakes on them. Make sure that they swivel 360 degrees, without hitting the plywood wall or catching anywhere else. 

Stand it back up and test the rollers. It should move well on concrete.

[Update- The casters could more easily be added when attaching the base/shelf.  Doing it earlier was definitely easier.]