If your Keynote exports are all fury and no sound, follow the instructions below to fix them.
After recording your narration, move to the Finder, Control-click on your presentation file, and choose Show Package Contents. In the resulting window, locate the narrationTrack0.m4a file. This is your narration track. Make a copy and move it to the Desktop.
Open both the narration and video files in QuickTime Player Pro. Select the audio track, select everything in it, and copy. Now select the video track and choose Edit -> Add to Movie. The audio track will be added to the video track.With luck, the two will be in sync. If not, you might wish, instead, to import the video track into GarageBand and then place the audio track in a separate track and move it around so the two are in sync.