Assigning Master Fader for Mackie Controller / SSL Matrix

I cannot find any details in the Cubase 7 manual or on line on how to assign the master fader on a Mackie Control (or an SSL Matrix)

it seems the master fader auto-assigns to the first physical stereo bus output of the sound card - whereas I want to assign it a software bus track (which feeds the physical output)

does anyone know how to do this?


The master fader will be assigned to whatever Output Buss is selected as “Main Mix” in the VST Connections window (F4 on PC) Outputs tab.