Firstly, Do you have the correct USB Driver Download - Yamahasynth.com
Secondly in Utility check that the Control/Midi Input selector is allocated to USB and the direction is in/out (I am presuming that this feature is the same as the Motif XS - if not I apologise).
3rdly you need to be in Song Mode for the Motif to react multitimbrally.
|The quickest way to do this is to create a song and make the program change/Mix data in the song and simply label the sequencer Tracks with the appropriate channels.
Once you have completed your Song Data Selection Bulk Dump the data into Cubase (Although if you are using SX Version 2 or below this will likely crash Cubase).
Alternatively you can control the ES with a setup bar including GM On although this is likely to reset the FX Insert Switches to a default Channel Setting of 1-8.
Another alternative is to load Studio Manager and see how you get on with that.
Monitor on headphones to check this is working correctly.
UNless you want to record the audio into Cubase (Why do you want to do this?) monitor this through your speaker system normally, or record it to a 2 track Master
(CD/DAT/ o whatever).