The extension was developed in 2015, therefore you need to use Slicer-4.5 or Slicer-4.6. Since then, Python, VTK, and Slicer API have changed considerably, but it should not be too hard to update the code to be compatible with latest Slicer release. It would be great if you could work on it and we can help if you have any specific question.
Right now I installed the 3D slicer 4.5. I could install the mentioned extension ( brachytherapy module) but I don’t have any idea how to use this. Have you tried to install and use this module?