all you really need to do on the max side is send the numbers 1-6 (and you also might want to add a number that turns all the LEDs off, e.g. 0). On the arduino side your sketch needs to listen for these numbers and when received turn on the appropriate LED (via WRITE, btw, not READ) by taking the corresponding output on the arduino HIGH. The ‘0’ I was proposing above would take all your outputs LOW. Send a message "0,5" message from max to turn off any LEDs that are currently on, and turn on 8,5,2, for example.
But again…not really sure what you are asking for help with.