In Mac OS X 10.5, go into System Preferences and add it as a printer. You may need to choose Generic as the printer type. (In Mac OS X 10.4, use the Printer Setup Utility to add it.)
Also from System Preferences (or Printer Setup Utility), add it as a Fax.
Open the Image Capture utility to perform scanning.
Mac OS X supports most of devices without any additional software. However, in some cases it may only be able to regard it as a generic type of device and some functionality may be simplified or not available. In such cases, it is up to the vendor (ie: Dell) to provide software to support the extra/advanced features. But unfortunately (and not surprisingly), Dell don't.