Here is how to assign the outputs. Go to Song Mix Mode. Press the Edit Button to enter Song Mix Edit mode.

When you first enter Edit Mode you are in Song Common Edit which has all of the parameters that are in common for all the Parts like Effects, Arpeggios, Master EQ, etc.

Press the 1-16 buttons to enter the Part Edit and select the Part you would like to send to the individual output.

If you push Part 5 for example, you will see a display of Part parameters for 5,6,7, and 8. Many parameters in Motif are arranged in "Groups of 4" (1-4, 5-8, 9-12, 13-16) because that's the way the sliders work.

Push [F3] to go to Output Select and set the output the way you would like, Finally, push Store and store the MIX.
This is on Page 211 of the manual.