If I want to select a multi I send a bank select with the first 2 digits of the multi preset number followed by a program change with the second two digits of the preset number to the global channel of the Origin.
If I want to select a program within a multi, I send a CC 0 with the first 2 digits of the prog preset number followed by a program change with the second 2 digits of the preset number to the channel corresponding to the given slot.
Hope that helps.