Select Missing Files Assistant… from the Slideshow menu if your slide is missing any media files. This assistant will help you to locate the missing files and repair the damage, provided that the original files are still available on your computer.

The assistant list all files that were discovered to be missing, showing a thumbnail, the original file path and the slide number where the files was used. Click on Locate to find and select the correct file and repair the damage. Click on OK once you are done.