To show photos one at a time, choose View ▶ as Slideshow.
To go to the previous/next photo, choose Photo ▶ Previous/Next, type ⌘←/→, hit the left/right arrow key, or click the previous/next toolbar buttons:
You can also simply click the photo to go to the next one. (If "Random order" is checked in Slideshow preferences, this will go to a random photo.)
To play the displayed pictures in a slideshow, choose Photos ▶ Play Slideshow, click the play button in the toolbar, or hit the spacebar. To pause, choose Pause Slideshow, click the pause button, or hit the spacebar again.
To go into full screen mode, choose View ▶ Full Screen. Hit ⌘` to toggle full screen mode. (You can also hit Escape to get out.)
You can set the delay between pictures in the Slideshow preferences.
If you check "Full screen", Shoebox will automatically go into full screen mode when you play a slideshow.
If you check "Random order", photos will be presented in random order. In full screen mode, hit the R key to toggle random order.
If you check "Show photos/thumbnails on second display", Shoebox will use dual displays in fullscreen mode. You can choose whether to display photos or thumbnails on the second display.
If you check "Loop slideshow", the slideshow will start over again when it gets to the end.