For a excruciatingly slow interpreter, you can just run main.py with Python.
It's recommended though that you use PyPy to compile to a native executable, which is much much faster.
brew install pypy
hg clone https://bitbucket.org/pypy/pypy $PYPY
export PYTHONPATH=.
pypy $PYPY/pypy/translator/goal/translate.py pumice/main.py
./pumice-c