Thank you very much for your help macropod and JohnWilson.

OK, I asked and it turns out that the Mac for the talk doesn't come with MS Office, it just comes with the built-in software. From what I'm reading, every Mac comes with Keynote, Apple's slideshow editor. It should be able to open a pptx file. Do you have any knowledge about this program? Might it alter the countdown timer or the embedded video?