Make a new Flash document. Draw a red ball. Convert it to a movie symbol called
'ball.' Give it an instance name: 'ball'. Add a layer on top. In it write this
script: this.ball.onLoad = function() { trace('Hello'); } Test the movie.
Nothing happens. Select the ball symbol on the stage. Hit F9. Type in: stop();
(or any other AS). Comment it out. Test the movie. It works. Why?

I should add that it is important to put the script on the selected symbol, not on the layer it is on. For some reason it doesn't work that way...