For developers and advanced users, anonymous SVN access is available from Google Code. Enter the following command line argument or SVN commands:
svn checkout http://music21.googlecode.com/svn/trunk/ music21-read-only
The easiest way to download and install music21 is with one of the powerful automated Python package installers available. These tools can also be used to update an existing music21 installation to the most-recent version.
First, install setuptools:
Second, install and/or update music21 with the following command-line argument:
$ sudo easy_install music21