This may be too simple of an example for some, but it shows how you can easily write a simple firmware for talking from Max to the arduino, specifically in order to control the motorshield from ladyAda. The max patch allows for control of all 4 motors and the 2 servos. Hopefully it is of use as a starting point.
I can't seem to get this running on my mega either, despite the fact that the library is supposed to be updated for it. Starting to wonder if I have mega issues....
Developed with help from David Fodel.