Make a slideshow in iPhoto 9.6.1 showing photos in random order
There is no preset for random order – but you can switch to random order ("shuffle order") while the slideshow is actually playing:


  "While a slideshow is playing, move the pointer to make the slideshow controls appear, and then click the Settings button" 


The settings button is the little cog wheel icon next to the close [X] button:



Clicking the settings button in play mode will open an extended preferences panel, featuring an additional option "Shuffle slide order" in the lower third of the panel:   



Checking/unchecking that option will toggle the Shuffle mode.

You can find a detailled description on this page "Change slideshow settings"at support.apple.com – it's for iPhoto 9.5 but should also apply for iPhoto 9.6.