As Nebula and Jimmy said, re-installing is 99.999% of the time not neccesary, and this sure isn't that 0.001%.
Make sure it's __init__.py (or pyc, also note double _ in front and behind init).
I'm afraid I made a mistake in my directions below (it was made from memory).
My appologies for the miss understanding, here are the corrected directions:
Check if the scripts/Custom/DurandalHP/__init__.py file exists and if the scripts/Custom/DurandalHP/Projectiles/__init__.py exists.