You can use the Preview function to see your video or publish it locally and watch it.
After embedding a video in a PPT slide, you'll just have a place holder. This can often be a white box, so it is easy to lose it on a white slide. If you go into Slide Show mode for PPT (F5) and go to that slide it should start playing and when you stop (Esc) the last frame of the video that was up should display on the slide. This can make it easier to see where the video is on the slide and place/size it appropriately.