First, download the zipped PyLSM folder from here. (Linked to from this page.)
Next, unzip the folder, navigate inside it at a command prompt, and run
sudo python setup.py install
The website for this python module is http://www.freesbi.ch/en/pylsm and there is some documentation here.
I haven't tried out the gui yet, but here is a snippet of code that should open a Janelia stack.