Frustratingly, the default install doesn't make PyMetrics executable, or place it in any convenient path for executing it.
I fixed this on my Ubuntu install by running:
sudo ln -s /usr/local/lib/python2.6/dist-packages/PyMetrics/PyMetrics.py /usr/bin/PyMetrics.py
sudo chmod +x /usr/bin/PyMetrics.py
However, due to the mechanism PyMetrics uses to locate metric modules, executing PyMetrics.py from an arbitrary location would usually result in failed imports of metrics. I've fixed this by modifying the __importMetricModules() function to test different import locations.