Don't have MX 2004 so this is the blind leading the blind, but...and this depends on how it was written , you are looking for the actionscript code which you should find in a tab called actions.
It may be blank when it opens up, that is because the code could be attached to an individual frame or an object on the flash movie 'screen', and it will display the code attached to the currently selected frame/object only.
So you will need to click on the objects and along the frames to find the code you are looking for.
Good luck