Binary package hint: gnome-session
Getting stuff added to autostart is much too difficult. First of all, guessing that it's in System->Settings->Sessions is not very obvious. Second of all, actually adding stuff to the list is close to impossible if you're not familiar with the layout of the Linux filesystem.
Possible solutions in order of preference:
1) Add it to the context menu in the gnome menu. If a program is set to autostart, make this obvious by putting a tiny emblem of some sort in its menu entry.
2) Add a new (sucks, I know) entry to the system->settings menu called "Autostart" or something. In there, present the list of stuff that's in the gnome menu and put a checkbox next to each entry to make it start automatically.
3) Keep the tab in the gnome-session-preferences, and replace the current "Add" dialog with a widget that allows you to select a program from the gnome menu and hide the "Add" dialog under some sort of "Advanced" thing.
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See also proposal by MPT: https://wiki.gnome.org/Design/SystemSettings/Proposals/LoginItems
Thanks for your report.