We installed this used furnace but can't seem to find were to hook up a 2-wire thermostat

There should be an R and W on the control board. Hook your wires to there.

there should be an r and w on the control board. Hook your wires to there.

That's what I thought but there is not I know you have to pick up 24v from the transformer but there is 3 wires already on the transformer and 2 extra post but none that indicates are or w

Sorry for the late reply, can you post a clear pic of the tranformer connections, and any other pics of the controls of the furnace would help also.