Answer:
There is no feature in Windows Movie Maker that allows for editing of imported media. In other words, you have to do all of your visual/audio clip-editing outside the program.
If you have imported an entire PowerPoint presentation into Windows Movie Maker without first making .jpg/.gif images of each clip, then the layout can't be changed.
When importing PowerPoint presentations to Windows Movie Maker, you should always save each clip as an individual .jpg/.gif file first. That way, when you import the media into the program, you can make adjustments to the layout.