If you, like me, have a Digitus USB to serial adapter that you want to get working under OS X you will have to download and install a driver.
At the time of writing, only Windows drivers are available at Digitus’ website, eventhough they claim to support OS X. However, reading their website I learnt that the device’s chipset is called FT232RL, and some Google magic reveals a list of FT232RL drivers. Simply download and install the appropriate one; I used the 2.2.18 x64 version for my Macbook running OS X 10.8.3. Once the driver is installed and the device is plugged into your computer you should see a new entry under /dev:
$ ls /dev/tty.usbserial* /dev/tty.usbserial-FTF6TGV5 $
$ screen /dev/tty.usbserial-FTF6TGV5 9600