I use the command line in Linux (and Raspberry Pi) to get permissions on files and devices.
What group does the Arduino display in your Mac? In the RPi, the Arduino Mega 2560 (/dev/ttyACM0) owner is root and the group is dialout. To access that device, the user must have dialout added to his/her user group. The user "pi" in the RPi has dialout group by default. (edit: I'm not sure about default because I installed an Arduino IDE in the RPi, so that may have added the group dialout to the user "pi" during that install.)
I don't have access to a Mac right now, but with Linux, open a command prompt and type "groups". It should display what groups your user belongs to.