To setup, visit Control Surfaces section in the Sonar preferences and add StudioMix, pointing to correct MIDI in/out (enabled them first in MIDI Devices section, if not done yet). If there is no StudioMix in the choice, try to install
https://www.cakewalk.com/Support/Knowledge-Base/2007013438/Control-Surface-Installers (I do not remember from where I have my).
But please note that old StudioMix units can:
* spontaneously move faders on there own, false thinking they are move by user. So it can slowly and randomly modify the mix (common problem)
* send broken messages, doi ng "random" things as the result (less usual)
* encoders/jogger generate in correct direction (aged light sensors?)
I have all 3 problems. But some people do not have them at all.
Note that faders are not touch sensitive nor precise. Please do not expect the quality and convenience of VS700 (the last is still close to $1000 on e-bay, StudioMix rarely hit $50).