newbie to mac and superduper!
 

scenario -- HD crashes dies -- Apple replaces it with a new HD, but we use this computer for business and we do have a backup copy of all of our files etc.. on the external firewire HD, but don't know how to put them all back on our computer?

All covered in the User's Guide -- "Recovering from a disaster".

In sum: start up from the backup, and use SuperDuper! to restore by copying back in the other direction. Alternatively, start the Mac with the new OS installed, and -- when prompted -- migrate from the backup.