For whatever reasons, D-Link saw fit to hold back their OS X drivers for the G122 54G USB dongle. The DWL-G122, however, uses a Ralink Technology 2500 series chipset. Just go to their Support page, and download the 2500 series USB Mac driver package. Install, plug it in, and you're done. You should get a notice telling you there is a new network interface when you go to your network settings. Select it in the second drop-down menu and configure it, and the dongle will then be added to the interface list.
One more important note: If you are using this on a system that already has a wireless card, like Airport Express, DO NOT RUN THEM BOTH AT THE SAME TIME. Two antennas that close to each other (less than 50cm) can actually cause damage to the cards. Turn off Airport when you're running the D-Link, unplug the D-Link when you're using the Airport.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050201091946184