As said above, you have to edit /etc/xdg/autostart/ and either:

• remove the NoDisplay=true lines;
• or make those same lines comments by adding # in front of them;
• or replace true by false in those same lines.

The third solution can be easily done in three steps:

1. Summon the terminal with Ctrl + Alt + T.

2. Type the two following commands:

   cd /etc/xdg/autostart/
   
   sudo sed --in-place 's/NoDisplay=true/NoDisplay=false/g' *.desktop
   

After changing anything you want, you can go back to the previous state by typing this into the terminal:

sudo sed --in-place 's/NoDisplay=false/NoDisplay=true/g' *.desktop

This has been tested in 12.04, 12.10 and 13.04．