Make sure the usb-serial adaptor is recognized by Mac OS X
You should see some items with Serial or PL in name if your adaptor is supported.
Then use following command to connect to serial port
screen /dev/tty. 9600
Then following vendor instruction to start password recovery process.
The break sequence for “screen” is to use Ctrl-a and then Ctrl-b. The Ctrl-a shifts the focus back to the screen process and the Ctrl-b issues the break. You might want to send the break several times by doing Ctrl-a, Ctrl-b, Ctrl-b, Ctrl-b…