I haven't used the KDE control center for adding in Samba users, but it sounds like there is a security permission issue preventing you from saving changes to the Samba setup. In the past, I've used SWAT for configuring Samba, or used the Webmin module to control access. If you're using Fedora Core 4, there's probably an rpm available for either SWAT or Webmin that will get in installed automatically and allow you to control it all from a web front-end. Shouldn't be that hard to get up and running.
The direct website for Webmin is
http://www.webmin.com, and even if you install by hand, it's a very easily install and shouldn't pose too many problems even if you're starting out in Linux.