This will often happen when you are using a browser other than Internet Explorer on PC. Installing the plugin for some browsers will make the video play, but often the slides will either not be timed correctly with the video, or will not work at all. You can view the slides within the presentation in these other browsers, but it will require you to click the "Thumbnails" view above where the slides would normally show. You can then click the icon that looks like a magnifying glass with an X through it to view a larger version of the slide. Note that the slides will still not be timed to the presentation, and thus it is recommended that you instead use Internet Explorer on PC in order to avoid these issues.