Have you reloaded the repository information after editing the sources.list using Software Sources? Software sources should present you a dialog.
It seems that we need a sanity check in gnome-app-install to fix this anyway. Or use software-properties --enable-compement for edgy+1 -- gnome-app-install duplicates universe component which was already enabled by 'Software Sources' in the Administration menu https://launchpad.net/bugs/64717 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
