A lot of people have been having issues adding new printers in ML. If you're sure the driver exists and should work but you're getting an error while adding the printer, you may have some success reseting the printing system. This, unfortunately, will remove all of your existing printers from "Print & Scan" in System Preferences, so I'd only do it if you're getting an error and you've tried everything else (including satcomer's suggestion above). That being said, it's a small price to pay to get it working.
Go to "Print & Scan" and, while holding down the Option key, click on the "–" beneath the list of printers. Select "Ok" and then, once it has reset, try adding the new printer. Good luck.