The most important part of the instructions is:
Copy living_hinge.inx and src/living_hinge.py to your extension directory.
Don’t forget that src/ is actually a directory you’ll need to add to the existing extension directory. Put the python script in the new subdirectory. Here’s the relevant bit from the Inkscape directions:
Copy the files into the directory listed at Edit > Preferences > System: User extensions. After a restart of Inkscape, the new extension will be available.
Finding the right directory is typically the biggest challenge.