Well, if there's ?s on the images, I don't think they're going to work with the slideshow. The ?s mean that Lr has somehow lost track of where the original image files are located on your harddrive. Usually this is caused by people moving or renaming the files outside of Lr, using Explorer or Finder.
To get started finding the images, either right click on a Folder with a ?, and choose 'Find Missing Folder....'. This will open a navigator to allow you to point to where the images are currently located. Alternatively, you can single click on the ? on a single photograph, which start a very similar process.
If this isn't what's happening, we're gonna' need more details about how your images came to have ?s on them.