One thing to keep in mind is SUS will only work with Windows 2000 SP3 and Windows XP SP1 or better. That may be why the registry locations are not appearing.
You will need to use a script to make registry changes on all the 2K/XP clients that you want to update with SUS, since you're not running Active Directory (where you could use a GPO to set it). The SUS guide from Microsoft outlines all of the registry keys and values you will need to make this work.
I have a script (it a .vbs script) that sets up clients, if you're interested, email me and I can send it to you (after you award the points, of course ;o) ).