If the Document inspector isn’t open, click Inspector in the toolbar and then click the Document button.

In the Document pane, select one or more options below Slideshow Settings:

• Automatically play upon open:  
  The presentation starts as soon as the Keynote document opens.
• Loop slideshow:  
  After the last slide, the presentation starts again with the first slide.
• Restart show if idle for:  
  If no user interaction occurs for the specified number of minutes, the slideshow begins again.
• Require password to exit show:  
  To quit the presentation, viewers must provide the administrator name and password for the computer on which the slideshow is being presented.