if you look closely there are 4 wires that come from the smoke unit that connect to the motor output joints on the RX13 board, this is the proportional smoke unit, the other two wires from the smoke unit go onto the 2 pin socket on the RX13 board underneath. I haven't got the RX18 board but it looks to me that you just plug in the smoke unit where it says on the diagram, the other 4 wires just De-solder from the RX13 board and connect to either side of the motors, make sure you get the wires right, same two from the RX13 board to the same motor and the same with the other two wires from the RX13 board.

When i replaced the RX13 board with another, i connected the proportional smoker wires to the motors direct, same thing really.

Kev