Using a flat head screw driver remove what's remaining of the old plug. Install the new plug, making sure you do not tilt the machine too much or it will size up.
One can either pull off the plastic fitting on one end (the one with an o-ring) and press the plastic tubing onto the pressure washer detergent inlet (located just behind the high pressure hose outlet/connector). The other end of the device is then inserted into a jug of pressure washer detergent and the lowest pressure nozzle or nozzle setting selected to allow detergent to be drawn up into the washwater stream with the pressure washer running. One can observe the detergent flowing into the washer if all is good! If one cannot pull off the plastic fitting on one end (the other fitting, to be left!, has a "screen" on its end - the fitting can be cut off the plastic tubing with a utility knife.