For anyone else who finds this via Google, the way to determine your port on a Mac is :
Open Terminal (cmd+space, type terminal, hit enter)
Type 'ls /dev/tty.*' without the single quotes obviously. Hit enter.
You should see a not too large list of ports that are in use. Mine looked like:
/dev/tty.Bluetooth-Incoming-Port /dev/tty.SOC
/dev/tty.MALS /dev/tty.usbmodem1442345
You're probably looking for one like /dev/tty.usbmodem1442345, but it will have a different number. If you have multiple ports that look like possibilities, just try them in pronterface.
Good Luck!