Install MIDI Library through the Arduino Library Manager (tested with 4.3.1).
If you are using the Arduino Uno, flash the USBMidiKliK_dual_uno.hex.build from USBMidiKliK.
Flash the .ino file using the Arduino IDE.
Connect the Arduino via USB cable. It will present itself as a MIDI device. When connecting the pins 2-13 to GND, a "note on" signal is sent, and a "note off" signal at disconnect. The analog pins 0-3 report analog values via MIDI CC.
When holding down buttons during power-up those buttons will change their behavior to a latching note-on/off.
The Board will, by default, not be programmable via the Arduino IDE, because it is now a MIDI device. To active programming mode, however, the smart USBMidiKliK firmware can set the board into a programmable state again, when receiving following MIDI sysex:
F0 77 77 77 09 F7
This can be achieved using amidi (installable under Ubuntu via apt):
amidi -p hw:0,0 -S 'F0 77 77 77 09 F7'
where you have to replace hw:0,0 with your device identifier (find out using amidi -l).
A simple reboot can be trigger by sending:
F0 77 77 77 0A F7
There are more sysex commands possible, check out the USBMidiKliK repo linked above.