That’s the command I use on Linux, OSX is similar:
avrdude -v -patmega32u4 -cavr109 -P/dev/ttyACM0 -b57600 -D -Uflash:w:game.hex:i
You’ll have to find where the avrdude executable is if it’s not in your OSX terminal shell’s path, it’s probably somewhere inside the Arduino.App directory (OSX apps are directories) and change /dev/ttyACM0 to whatever /dev/something the Arduboy shows up as (the Arduino IDE will tell you)
Alternatively I think Team A.R.G. made an OSX version of their uploader tool if you prefer that.