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