In reply to Heike:
You should be able to do this on most current versions of powerpoint. What version of PPTX have you got and have you got any room on your MAC's HD
1. Make your presentation
2. Go to the slide Show tab
3. Press record
4. Narrate your slides
5. Save as powerpoint or QuickTime movie depending on the final output needs.
We do this a fair amount and there should be no problem. It is much more clunky than ECHO, Stream Capture or Canvas, but it works OK.