For the normal spellcaster version, They add all witch spells with the curse descriptor to their list. I was looking through the witch WoP list and can't see any definitive "Curse" words. What effect words would be added to the list?
They wouldn't be.
You would get the actual curse spells as normal. You could cast your curse spells per the normal rules OR alternate to your normal word spells as needed.
It functions just like a sorcerer's bloodline bonus spells or a witch's patron spells, you get the actual bonus spells along with your words of power components.