ReLaTeX is a pure Python package; you need Python to run it. You can either just run the downloaded relatex.py script directly (no install step required), or you can run its setup.py script to install it on your system. Both options have advantages and disadvantages:
You can just run relatex.py directly like this:
python /path/to/relatex.py plos my.tex
advantage: this way, it knows where its templates directory is (in the same place where relatex.py is), so you can specify a standard template just by giving its name, e.g. plos.
disadvantage: ReLaTeX requires the Python Jinja2 package. You either have to have Jinja2 already installed, or install it yourself via your system’s software installer, or Python’s mechanisms such as easy_install and pip, e.g.:
sudo pip install Jinja2
(leave out sudo if you are installing to a location you have write privileges for).
In the downloaded relatex directory run:
sudo python setup.py install
(leave out sudo if you are installing to a location you have write privileges for).
advantage: assuming your Python has setuptools, this will attempt to automatically install Jinja2 for you, if it is not already installed.
disadvantage: you will have to specify the path to your desired template file, e.g.:
relatex.py /path/to/relatex/templates/plos my.tex