htat's the one I'll bet you. It's hard for me to tell exactly, but if it is like mine, here's how it works. Roll out your backwash hose, turn off your pump (don't move the valve with the pump on), turn the handle 90 degrees, then push down on it toward the ground. This will probably put it in backwash mode.
I was just reading that some are pulled up instead of pushed down, so try each.