That is strange. You should make sure you have the Extras repository enabled, but not Extras-Testing or Extras-Devel. Once you have verified that, update and make sure The Ur-Quan Masters isn't in your list of downloads one more time.

If you still don't see it, you should try using the apt-get utility from the command line. Make sure you have rootsh installed, close App Manager, and then open an xterm (Ctrl-Shift-x) and type If that doesn't solve the problem, please let me know what output the apt-get command gives you.