To watch the Slideshow manually, choose the picture from the thumbnails, and use the advance button (directly to the right of the "+" button) to advance to the next set of pictures.
If you would rather sit back and watch the entire Slideshow, click on the play button (the triangle pointing to the right), and the pictures will automatically scroll.
For the ultimate experience, choose the "full screen" icon next to the play button and watch the slideshow in full screen mode. You will need the latest Flash player to view full screen mode.