To add folders or iPhoto/Aperture libraries to the list of locations to scan for duplicate files, do one of the following:
Click the plus button below the folder list and choose one or more folders, iPhoto or Aperture libraries using the standard File Open panel. Hold down ⌘ while clicking to select multiple items.
Press ⌘O and select folders or iPhoto/Aperture libraries using the standard File Open panel. Hold down ⌘ while clicking to select multiple items.
Drag and drop folders or iPhoto/Aperture libraries from Finder onto the folder list.