So if I change the firmware on it, I first need to write the code? If I understand it correctly. Such as I write the code for what I have lets say 10 buttons and 2 pots, I write my code for it put it on my arduino uno then change the firmware to what you sent me?
Yes. So the steps would be:
1. Work on the sketch
2. Write sketch to UNO
3. Put UNO in DFU mode and flash the MIDI firmware
4. Test to see if it works
5. Not working properly? Put UNO into DFU mode and flash serial firmware. Goto 1.
If you have an FTDI serial to USB cable then you can connect that to a couple of digital pins on the UNO and use software serial to provide some debug output.