Click Options menu to open Options Settings window.
Set Play Slideshow property to either Random or In Order.
Click the + button to open slideshow transitions window.
To enable slideshow, should leave Layout Settings > Slide > Width, Layout Settings > Slide > Height and Layout Settings > Slide > Align unset.
See Layout Settings.
In the slideshow transitions window, select transition to preview, click add button to add transition, click remove button to remove transition.