Music21 can be installed in several different ways. Advanced users can install music21 as they would any other python package. See the link to advanced instructions below. Other uses should follow the instructions for each platform by following the links below.
Mac users: Installing Music21 on Mac
Windows uses: Installing Music21 in Windows
GNU/Linux uses: Installing Music21 on GNU/Linux
The above instructions are for general users. If you are an advanced user and have installed other Python packages before, or want to use an EGG, SVN, PIP, or setuptools, you should read Installing Music21 with SVN, setuptools, and PIP.