It took me a couple of Googles to work this out... I have a Belkin f5u103v USB-to-Serial adapter and needed a console connection to a Cisco switch.
To get started install this driver from apple (I think it needed a reboot).
If this was successful when you connect the USB-to-Serial you'll be asked if you want to setup a modem / network connection... say no. From a terminal you should now see a new device similar to mine...
NickBook:~ nick$ ls /dev/cu* /dev/cu.Bluetooth-Modem /dev/cu.PL2303-00001004 /dev/cu.Bluetooth-PDA-Sync NickBook:~ nick$
Since I don't want to re-invent the wheel now go to http://www.macosxhints.com/article.php?story=20040521145713551 and Start at STEP 3 to configure minicom (Obviously you'll replace /dev/cu.USA19QI191P1.1 with something similar to /dev/cu.PL2303-00001004 ) once finished you'll be set.. happy terminal session!