If you are lazy, just perform
$ pip install plac
which will install the module on your system (and possibly argparse too, if it is not already installed).
If you prefer to install the full distribution from source, including the documentation, download the tarball, unpack it and run
$ python setup.py install
in the main directory, possibly as superuser.
$ python doc/test_plac.py
$ nosetests doc
$ py.test doc
Some tests will fail if sqlsoup is not installed.
pip install sqlsoup or just ignore them.
The source code and the documentation are hosted on GitHub. Here is the full documentation in both HTML and PDF formats: