_Mathematica_ searches a local user directory (see $UserDirectory) in addition to the system-wide location for add-on packages. The musictheory/src directory needs to be added to this path. This can be done in two possible ways:
1. Create a symbolic link in $UserDirectory to the musictheory/src directory. Using Terminal:
cd ~/Library/Mathematica/Applications
ln -s ~/musictheory/src ./MusicTheory
2. Add the path to the list of trusted directories:
- Go to the menu "Mathematica > Preferences"
- Select "System" tab
- Click "Edit Trusted Directories..."
- Add an entry corresponding to the location of musictheory/src on the system.
Mathematica may need to be restarted after changing the path configuration.