Um, this is kind of embarrassing. I found out after some more digging that I wasn't in KDE at all. (That would explain why the KDE menu editor wasn't working....) I was in Redhat's default Gnome desktop which, incomprehensibly, has the menus set to be uneditable. (Bumped into this tidbit in "Fedora Core 3" by Ball and Duff.)
They point out that changing /etc/gnome-vfs-2.0/modules/default-modules.conf to the alternative file solves that problem. (Rename default-modules.conf.with-menu-editing to default-modules.conf after saving the original default under a new name like default-modules.conf.without-editing.) Well, yes, it worked, but the new default has no menus at all. One of my biggest problems--as with most newbies, I suspect--is just finding the damn files in Linux, so without menus I'm fairly hamstrung. I wanted to edit the system default, but that had disappeared.
So, anyway, that didn't really work for me, but at least I found out I wasn't in KDE. Once I switched to KDE, then, as you say, menu editor worked just fine.
All's well that ends well, right? Thanks for your reply!