hit the PROGRAM key and choose your patch
hit the EDIT key
hit your left MORE soft button until you see VTRIGG
(which stands for velocity trigger)
hit that soft button and it will bring you to 4 options
Vtrigger 1
Vtrigger 2
Vtrigger 3
Vtrigger 4

assign them to taste and exit -
it will ask you do you want to save... say yes!
and you are on your way.