Sorry for not looping back here in January. I had not touched Will-777's code in a while.
I'm at least able to generate an mcpi_mobs_mgr object when following the instructions:
Just Launch it with right click and "open"/"Python2 IDLE" You should have a window with the code. Then press F5 to launch it
It appears you skipped that step?
If you prefer not to use IDLE, it's also possible to run straight from the command-line Python interpreter via
git clone https://github.com/Will-777/spawn_mobs_mcpi.git
then continue his instructions from there.