i was having the same problem; nothing would move, no ships would spawn etcetera. I changed the ship instance from 'Ship' to 'ship' and everything worked perfectly. only took me an hour to work out that it was a capital. :(
It means you don't have a movieclip with the class as missile.when you convert it to symbol, go advanced otions click import for actionscripting, then make the identifier and class missile and it should work.