I should try this on a Windows machine, but I believe the steps are:
- Copy the ae_to_spine.jsx file to the "Scripts" directory under "Adobe After Effects CC 2014" (or 2015, etc.)
- Copy the "lib" directory to the "Scripts" directory as well
In After Effects, click File | Scripts | Run Script File...
Navigate to that "Scripts" directory and open ae_to_spine.jsx
Click [Export] on the UI that appears.