It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.entity.IEntityArrowTipped;

IEntityArrowTipped extends IEntityArrow. That means all functions available to IEntityArrow are also available to IEntityArrowTipped.

• void setPotionEffect(IItemStack stack)

• void addPotionEffect(IPotionEffect effect)