Intall Perian, http://perian.org/ which should let you play it in Quicktime. You maybe able to then import it into iMovie, or do a Save As from Quicktime and then use iMovie and add the audio.
Still, I have no bloody idea why degrade and recode the original video in a time-consuming process, when all you need is add another audio track to an existing file. Which mkvtoolnix does just brilliantly.