Tip for Reaper users, on how to set up the Model D. Took me several days to figure this out, hope it is useful to you. I'm using an M Audio Projectmix soundcard, got the audio output from the Model D routed into input no. 7 (mono).
Arm track record, and right click on the red recording button:
Monitor Input
Record Output: mono
Input: mono - Projectmix 7
Midi hardware output: Model D, send to channel 1
Make sure you have the Model D connected to your DAW / soundcard with the USB cable, and enable it through Options > Preferences > Midi Devices.
And you're good to go.