How to pass a parameter to a MovieClip

I have a flash application (the main movie) and in it a movieClip (I call it my_dialog).
When the main movie starts, I want to pass a parameter (the user’s username) in to the movieClip (code in main.fla: my_dialog.username(“panoss”)).
In my_dialog I have made a function called ‘username’. But it says that the username is ‘undefined’.
I think the problem is that the movieClip ‘my_dialog’ has not yet been loaded at the time of the call. What can I do;