How do I increase the laughter on my comedy DVD?

If the audience was recorded on the same microphone as the talent then you can't do much, if anything, in iMovie to boost what amounts to ambient sound. You could use some of the 'laughter' tracks from iLife Effects but then they all sound the same. Not very spontaneous but it might work.

Maybe you could record a bunch of people watching your DVD and use that soundtrack.

There may be some audio programs that can do that sort of thing, and perhaps there are others on this forum who will chime in . I don't know of any myself.

If I were doing this just using iMovie, I would extract the entire sound track (Advanced/Extract Audio) and drag it to a new iMovie project. Then, in the new movie project, I would Select/Edit/Copy the sound track and paste it multiple times one on top of the other. Each time you do that, the volume will increase. When you get the applause portion volume to where you want it, export the soundtrack to your desk top as an .aiff file. Then import it into your initial iMovie project and mute the performer's voice parts. The louder applause parts will then be inserted into your movie.

You can add laughter as a sound effect. I found 'Indoor chuckle' and 'Indoor laugh' in Skywalker Sound Effects in iMovie's Audio.

Just drag that laugh clip down to the timeline audio track and trim it to the length you want:
Place the playhead where you want the laugh to end.
Click on the clip to highlight it.
Command-T splits the audio clip at the playhead.
Click on the unwanted part to highlight it.
Delete.

You create a new iMovie project so that its window is side by side with the initial project. Then you either drag the sound track from one to the other or you Edit/Copy/Paste it.

But if the laughter sound effect will work, that would be easier. You would need to try it to see if it sounds artificial.