OK. Here is what I did.
Connecting MIDI Designer Pro 2 on an iPad Pro to Digital Performer 9.3 on a Mac using USB:
1. Install MIDI Designer Pro 2 on your iPad
2. Download musicIO server on your Mac (available at http://musicioapp.com/downloads/)
3. Connect the iPad to the Mac using a lightning charging cable, insert the USB end to your computer or hub.
4. Run musicIO on the Mac.
5. Run musicIO MIDI on your iPad.
6. Both the iPad and the Mac will indicate that they are Connected (the word “Connect” inside a large green circle. On the Mac be sure that the “Display” tab is selected.
7. Run MIDI Designer Pro 2 on the iPad
8. (Optional) run Vienna Ensemble Pro Server 6.0 15864 (64 bit) latest version. That's what I use. Remember to launch VEP6 before opening DP9.
9. Run Digital Performer 9.3 (latest version)
10. In DP9 go to the “Select” menu and select “Control Surface Setup” The Driver and Unit boxes should be set to “Mackie Control”.
11. The Input Port should be set to “musicIO-iPad-1”.
12. The Output Port should be set to “musicIO – iPad”.
13. In MIDI Designer Pro hit the “More” button and select “Config” and at the bottom of that window, select “Connections”. You should see your computer’s name under where it says “Wi-Fi Connections”
14. Tap your computer’s name and it will flash and you will very briefly see a check mark to the right, then it vanishes…but don’t worry…all is good.
15. Select “More” in MDP2 and select “Design”
16. Create a slider (fader) and double click on it.
17. At the bottom of that window syou will see “Properties”. Select that and you can name the fader (or knob…or…). Select “normal” under subtype.
18. Select the MIDI icon at the bottom of that window. The top box is where you can set the channel number and the CC number. For volume, set this to CC7.. Message type should be set to Control Change.
19. Exit design mode.
20. The fader or knob, or whatever you have created should now control the DP9 fader in the Mixer window for any track set to MIDI channel 1.
Good luck. I probably have much more experimenting to do.