If I remember correctly, there are a couple of different ways you can set it up in the m-unit config, but the default is to toggle between the two every time the light wire is grounded. A momentary push button (like the style your horn or starter button uses) would be the easiest. If that's not a preferred option, wire the headlight main wire to the light input on the m-unit and then wire the output high from the dimmer switch to ground. Low remains unconnected to anything. Now, whenever you switch to high beam and back, it will change between high and low.