You can't do that in Powerpoint you have to edit the video with a video editor.
What difference does that make? Same issue Mac or PC. PowerPoint is not a video editor do you have one?
This will work: http://filmora.wondershare.com/video-editing-tips/fast-and-slow-motion.html
Edit the video using that then reimport to your Presentation. If you are satisfied, please kindly provide an OK or Above Rating - so I get paid. Keep in mind you can always come back to this page at any time regarding this question. Thanks!!