What Ms. Lucy Said.
And:
Mac: How do I recover corrupt or unreadable PowerPoint presentations?
http://www.pptfaq.com/FAQ00423.htm
Recovering a corrupt presentation
http://www.pptfaq.com/FAQ00108.htm
And if you can move the file to a Windows PC with any 97-2007 version of PowerPoint, try opening it there. If that works, save the presentation (as PPTX if you've got Mac Office 2011; as PPT if you have an earlier version). Try opening it on the Mac. If
that still doesn't work, trundle back to the PC and thump the presentation with this:
HTML "Round-tripping" to repair corruption
http://www.pptfaq.com/FAQ00526.htm
Steve Rindsberg PowerPoint MVP
PowerPoint FAQ
PPTools Add-ins for PowerPoint