I found this out quite by accident I just got done reinstalling Ubuntu 12.04 I got the operating system up and the program Software Sources was installed but could not put it in the Unity panel. So this is what i did to put it back in the panel.

1st: Go to Update Manager at the bottom left of that program that comes up is a button called Settings

2nd: click on settings and Software Sources should come up in the panel.


3rd: Then goto the Icon in the panel right click on it and lock it to the panel.



there ya go