It has been a while, but I seem to recall USB to serial bridges being /dev/tty*. The easiest way to find out what the * is will be to unplug the dongle. From the terminal type 'ls /dev/tty*'. Plug in the dongle. From the terminal type 'ls /dev/tty*'. See what new /dev/tty devices you have. Wait a minute or so after unplugging or plugging in the dongle before typing in the command so that driver has time to detect the changes.