Sorry moderators... can you erase this thread? Question already solved... =P
magic missile is a target spell... to cast a target spell you must be able to see the target (Core Rulebook page 214).
Note that unlike say an invisible stalker, there is nothing about the will o' wisps natural invisibility ability that says it can attack and remain invisible. Because they have no ranged attacks, they will often have to move to get into place to make an attack, meaning that they will not be able to go invisible again after that attack (because going invisible is a move action for a will o' wisp). So in a fight there are usually going to be times when casters can target the creature with magic missile or maze spells.