Hi.
The UsbMidiKlik firmware maps USB Midi to midi serial (bi-directional).
You can then use you own serial midi parser (or Midi Library) in a classical Arduino sketch with the benefit of having a HID USB Midi compliant device. Don't forget however that you have only one serial IN and one serial OUT with an Arduino Uno. The uC managing USB (AT 16u2 ) is connected directly to the serial I/o pin of the Arduino : you can't mix signal from serial midi and USB...
If you need a small footprint midi parser, look at the midiXparser at the Github. It is the one used in the USBMidiklik firmware.