Well I seemed to have the same problem with a new installation of FC2. First I could edit, then things started moving on their own and deleting, then it eventually just stopped letting me edit it.
I found a "defect" in the KDE.org database. I followed one of the solutions and it works!
Check Additional Comment #3. That's the solution I followed.
Incidentally, KDE considers this to be a "wishlist" item, though the fact that you don't have a usable menu anymore - to me - seems a bug. I recommend that anyone using this fix log into the KDE site and put some votes towards that item.... after all the more people who want it, the more likely it will get fixed!
P.S. - I didn't run the kmenuedit in the middle, and the FC2 location of the xdg menus is right off of /etc/xdg......