If you’ve upgraded your system to 10.10, when you look in System > Administration for Software Sources, you’ll likely find it missing. But is hasn’t been removed, just hidden.
Simply right-click the System menu, and choose Edit Menus. In the left hand pane, go to the bottom and select Administration within System. In the right-hand pane, simply check Software Sources, then click Close.
If you find you have two of them listed, that would be because you have KDE as well as Gnome in the one system. All you have to do to find out which one to enable is right-click them and choose Properties. The KDE version will have the command as something like software-properties-kde, while the Gnome one will be gksu
--desktop /usr/share/applications/software-properties-gtk.desktop /usr/bin/software-properties-gtk or similar (the important point being the one with “kde” in the name is likely not the one you want).
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