Click the Movies icon in the toolbar to reveal the Movie Browser in the sidebar.
The list at the top shows your Aperture, and Adobe Lightroom libraries, or any Finder folder containing movie files. You can exclude some of these libraries or folders by unchecking the corresponding items in the Preferences window. Drag any folder that contains movies to the list to permanently add this folder to the list. Optionally you can right-click on the list background to show a context menu and select the command. Remove the folder again by selecting from the context menu.
The movies of the currently selected library, album, or folder are displayed below the divider line. Drag the divider line up all the way to collapse the list of libraries to a small popup menu. This maximizes the amount of vertical space available for the Image Browser. This is especially helpful on small notebook displays.
Select one or more movies in the browser and drag them to the Storyboard or Stage to add them to your slideshow.
If you want to search for specific movies, type a word into the search field below the Movie Browser.
Select a movie and press the Quick Look button or press ⌘Y or choose the command from the context menu in the browser to open the Quick Look panel. You can then quickly preview a movie. Press the Quick Look button or ⌘Y again to close the Quick Look panel.
Drag the size slider to enlarge or decrease the size of the movie thumbnails displayed in the browser. This slider is not available in list view.