Beagleboard C4 needs conversion from the IDC10 pin header to D-Sub 9 Female cable.
Header - Connector
Ok, then you need the USB2Serial converter. I've got Digitus, which on Mac uses Prolific PL 2303 driver, downloaded from here. (EDIT: Prolific.tw site is crock, their drivers don't seem to work with Mountain Lion ATM, download FTDI virtual com driver instead)
To get to the serial console, find your usb2serial device
My device is displayed as:
/dev/tty.usbserial-FTFE6UC8So, use screen command:
screen /dev/tty.usbserial-FTFE6UC8 115200