Let us just say: if we had to pick something to add to the list of the five "normal" senses (sight, hearing, touch, smell, and taste), we probably wouldn't go with Cole's particular talent. Instead, would might add something more along the lines of…

• The ability to turn roaches into puppies
• Knowledge of how magicians saw people in half
• The power to turn anything into chocolate

Even just garden-variety psychic powers, which are often referred to under the "sixth sense" umbrella, would be okay. Unfortunately, Cole's powers aren't any of the above; instead, he gets to communicate with dead people.

So you don't need a sixth sense to understand how they decided the title for the film. Cole has a special ability to see the spirit world, and that power drives both his story and, by extension, Malcolm's.