I’m using the plugable USB to RS-232 DB9 Serial Adapter here: http://plugable.com/drivers/prolific
Download the Mac OS driver and install it. You will have to restart system when the installation is completed.
Once your system is rebooted, plug in the adapter to your Mac and the RJ-45 connector to the console port of the Cisco router or switch.
Run following command and make sure that the adapter shows up, e.g:
$ ls -ltr /dev/*usb* crw-rw-rw- 1 root wheel 18, 2 Mar 19 22:28 /dev/tty.usbserial crw-rw-rw- 1 root wheel 18, 3 Mar 19 22:28 /dev/cu.usbserial
Run following command to start a console session:
$ screen /dev/tty.usbserial 9600
To disconnect the session, enter Ctrl+A followed by Ctrl+\.