First, make sure that you have the most up-to-date Quicktime Player.
Second, make sure that when you hover your mouse over the “camera” icon at the bottom of your screen you can see a text description “There is a movie to play here”. This indicates that your system recognises that there is a movie to be played for you.
Third, click on the small triangle button, so to play the movie.
If you still can not see the movie, use a different movie theatre that plays the same movie (that is provided in a different region in SL). Sometimes, an update/upgrade of the underlying simulator that renders a region in Second Life may cause some users to have problems.