When I use Arduino with chip-related or hardware related stuff, I often need to send MIDI data to the board. Sometimes I can't be bothered making a MIDI input circuit for the Arduino, or it may be beneficial to run a USB line for other reasons (5V, for instance).
In this case, I use this very basic patch. Simply load it up in the Max 5 environment, and then select your MIDI input device (usually a virtual path) by double-clicking on the midiin object. If you're in Windows, you may need to edit the patch. If you are on a Mac, and your Arduino is not serial port A, you will need to edit the patch.