This should be in the iPython music21 documentation somewhere, my apologies: iPython in music21 requires Lilypond to be installed for images to be generated within the notebook itself. There hasn't been (and won't be until MuseScore 2.0 is released) a way using MusicXML to generate PNG images of scores directly.
Edit: 2015 July; music21 2.0 w/ MuseScore 2 will generate the PNG images with MuseScore if it is installed and fallback to Lilypond if it is not installed.