I like options, so here are three:
You can export from Powerpoint as a Quicktime movie. I've had mixed results with my older version of Ppt, but see how your's turns out. To add the voice-over, import the Quicktime movie into iMovie. There you have the option of recording another audio track.
You could also do this the old school way -- project your Ppt slideshow onto a screen and use a camcorder to record both the slideshow and your commentary.
Finally, you can use software like SnapZ Pro to capture what's on your computer's screen (like a screen shot, but it captures everything as video). You'd then use iMovie or Quicktime Pro (or other software) to record/add your audio commentary.
John