For USB MIDI, you could use V-USB, a software-only implementation of a low-speed USB device for AVR micro controllers.
Check that :
http://morecatlab.akiba.coocan.jp/morecat_lab/MOCO-e.html
If you have a lot of controls, better to buy an Arduino Uno,the one with the ATMEGA8U controlling the USB, then upload an USB midi dual firmware like mine GitHub - TheKikGen/USBMidiKliK: A robust USB MIDI Arduino firmware, with a dual bootloader, based on the LUFA library