When you right-click on a folder with the question mark, you have to select <Find Missing Folder>. Then navigate to the correct location of the folder and select it.
With individual images the procedure is a bit different: You left-click directly on the question mark on the frame of the image and select <Locate>. Then navigate to the image and select it.
For more see here:
http://tv.adobe.com/watch/creative-suite-podcast-photographers/lightro om-dude-where-are-my-photos/
and here:
http://www.computer-darkroom.com/lr2_find_folder/find-folder.htm
