First off, I'm putting in a disclaimer:
I'm not a developer or employee of ADJ or Elation.
That being said:
The MIDI control is "scene/trigger" based only. You're NOT given the option to use a MIDI device to use as a CONTROLLER.
In my experience with testing, I only have devices for the most part that are only giving me note on/off control, mainly since if I bring a keyboard into the house, my wife will kill me. Too big, so I've been using a dinky MS-1 sampler. It has been shown that MyDMX will work off of other controller input, namely rotary pot cotrollers, and I suppose that it could work with volume controllers too.
The MyDMX program simply listens for any MIDI input on any channel. I know for sure it can listen to and respond to note-on, which is the easiest for triggers.
Ideally, if you want that sort of control, you need to go with a software application that can work in conjunction with a DMX controller. I mean, it's one of those "right tools for the right job" sort of thing.
So, just to clarify: MIDI implementation is based on scene triggering, not design control.
Make sure your rotary pots are not set for "local only" and they might work, but I wouldn't want to use those as triggers.