Well I hope we're not doing your homework for you but...

As putStrLn is in the IO monad, listFilms should at least have a signature like this:

listFilms :: [Film] -> IO ()

Now you know you want to call putStrLn on each item in the list. That means you want to join a bunch of functions return IO (). That sounds like a job for sequence_:

listFilms films = sequence_ $ map (putStrLn.show) films

Or the same thing more verbosely:

listFilms [] = return ()
listFilms (film:films)
 = do putStrLn (show film)
      listFilms films

Hope that helps,
Alex