There is a lot of things that could be going wrong here.
1) Is the arduino board an actual genuine/"genuino" board?
2) What type of arduino? Uno?
3) When you say its discoverable, in what way do you mean?
4) Are you sending MIDI commands from the Arduino? If so, what type of commands are you sending. MIDI syntax can sometimes be a pain.
5) Lastly How is the arduino interfaced to Watchpax machine? Windows requires specific MIDI drivers for a MIDI device. If you are interfacing through the USB cable then you need to use the MIDIUSB library for Arduino, and you also need to read the article here. I'm not familiar enough with the embedded Windows environment on Watchpax to know if this setup will even work on USB. It may not. In that case, I would suggest getting an official MIDI input device and sending your commands from the Arduino to that device.
