Here is the easy way to embed a video in PowerPoint or Keynote:
  1. Simply drag the file into your slide, done.
  2. Resize the video so that it fits neatly on the slide.
  3. Play the video until you hit the scene that you want to use as your place holder image. Hit pause.
  4. In PowerPoint: choose format movie, poster frame, current frame.
  5. In Keynote: select the movie, go to the inspector, hit the far right Quicktime icon and slide the poster frame slider to the desired location.
There are more advanced control features available, but these basics should do for most users.