When your printer is on and connected to your Mac via USB, 2 USB ports should be listed next to the Connect button, xxxxbluetoothxxx and xxxserialxxxx. If the xxxserialxxx is not listed, you may not have the USB-serial drivers loaded. Go here and download and install::
http://www.ftdichip.com/Drivers/VCP.htm
I can't remember if the Mac needed rebooting afterwards, but it worked for me.