Opponent cannot move with IMac???

My opponent is on an IMac and cannot seem to move a piece. Could there be a glitch??? I told him about the "Submit" button and how to click and drag the piece; however, neither of these solutions are working. Could there be something I am not thinking of, or could it be an IMac problem???

You can go to my opponents and look up "strengthcoach" if you want to help without going through me.