installing ms-ops2 in old house. There is no ground wire. What to do?

There were 3 black wires on the old switch. One connected to one end of the switch and two black wires attqched to the other end. But no ground wire. I tried wiring the single black wire to a OPs2 black wire and the other two old black wires to the other OPS2 black wire. Of course it didnt work. Any suggestions?

It needs a voltage reference. The metal box should be grounded. Try connecting the ground to the box.