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 trying to trace it in the my_dialog, the output is 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;