First of all, you will need to install the free DivX player (see main page for link). We have done so both under Windows and Mac OS X. The videos won't play from within Powerpoint if they don't play from a standard video player on your platform.
On some platforms, it is necessary to disable graphics card hardware acceleration in order for videos to play. You can do this from the Display section of the Control Panel (follow Settings/Advanced...). I have seen this problem in some cases under Windows, but not always.
The videos play fine from within Windows Powerpoint on our system. However, if we open the ppt file under Mac OS X (in the Mac version of Powerpoint), the videos don't play any more. If we manually re-embed the videos in the Mac version of Powerpoint (slow process, as one has to manually resize the videos in place on every slide that has video) the videos play fine, but then the same ppt doesn't play under Windows any more. The ppt that we provide in the zip file plays fine under Windows on our system.
It would be greatly appreciated if these incompabilities between different platforms were addressed in future versions of Powerpoint. I find Powerpoint most convenient for my presentations, and having to re-embed videos when switching platforms really slows me down. Or, please provide a menu command to re-locate the video; this way, at least it won't be necessary to resize and reposition each video.