The Machinedrum MIDI machines send MIDI information on MIDI channel numbers that are equal to the Machinedrum track numbers, so that track number 9 only transmits on MIDI channel 9. This is described on page A-12 of the MD manual.
Edit: JSZ pointed out in a later post that this is not true.

On the Nord Lead 2X, you need to set the MIDI channel numbers of the four slots to the same numbers as the MD tracks MIDI channels on which you are using MIDI machines.

For example, if you have MIDI machines on tracks that use channels 9, 10, 11, and 12 of the MD, set up the NL2X so that slots A, B, C, and D are set to MIDI channels 9, 10, 11, and 12 respectively.

The MD can sequence three notes at a time on each track and you can use parameter locks to sequence different triads of notes on each step of the sequencer.