Etapas que fiz para instalar libxml2 no mac os 10.9:
# install the library using brew
brew install libxml2
# go to python site-packages folder ( in this case, python 2.7 )
cd /Library/Python/2.7/site-packages
# link all python files from site-packages from brew to current python
# ( in this case, libxml2 version is 2.9.1 )
ln -s /usr/local/Cellar/libxml2/2.9.1/lib/python2.7/site-packages/* .Preste atenção na site-packagespasta certa se estiver usando o virtualenv.