Reply to "Aux" Volume knob.....

I recently got a Stanton DJC4 and I want to map the headphone knob to control the headphones volume and when I hold shift it controls the "AUX " level. Here's the problem, I can't find the aux volume in any of the drop Downs in the controller manager thingy. Can it even be mapped?? If it can What's it's called??

Thanks for the time and info.....