here's my setup for mac os x + max/msp:

- followed all the steps at http://www.arduino.cc/en/Main/Howto
- downloaded SimpleMessageSystem code from 
  http://ciam.dyndns.org/~vitamin/danslchamp/mrtof/SimpleMessageSystem.zip
- followed the installation instructions for SimpleMessageSystem in the read_me.pdf
- tested the max/msp example, SimpleMessageSystem.mxb, and modified the 
  arguments of the serial object in max/msp to specify the arduino board: 
  "serial usbserial-A1000gec 9600 8 1 0".

to find the serial port device in the arduino ide, i followed the Howto instruction, 
which suggested it would be something like '/dev/cu.usbserial-A3000Yj0', which you 
select from from a list in Tools > Serial Port.

in max/msp, i found the port by sending the 'print' message to the 'serial' object.  
this gave me a list of serial devices on my computer, one of which resembled the 
'usbserial-AXXXX' from the arduino ide. 

-Owen