First you should install pymodbus with easy_install, pip or from the cloning repository:
easy_install -U pymodbus
pip install -U pymodbus
Otherwise you can pull the trunk source and install from there:
git clone git://github.com/bashwork/pymodbus.git
cd pymodbus
python setup.py install
GIT repo for pymodbus is here : https://github.com/bashwork/pymodbus
API Doc for pymodbus is here : https://pymodbus.readthedocs.org/en/latest/library/index.html