This doesn't surprise me since support for Python 3.x is spotty at best (when you look at the module ecosystem as a whole). It also doesn't surprise me that it works with the system Python since that's Python 2.x.

I would imagine that the version of Pygame from MP would work with their build of Python. Thus, I wonder if you're using the binary installation of Python 3.x that you have installed without knowing it. MP installs its content to /opt/local so execute a "which python" to see what version of Python you're defaulting to. If you're defaulting to the Python.org (official) version, which will be installed to /Library/Frameworks/Python.framework/Versions, you can install the "python_select" tool from MP to switch the default.