PLEASE HELP - How do I Intialize a movie clip when Onload doesn't work

I have movie clip (called MainButton) that I am using as a button and it has an ActionScript function (called Load_Data(str:String) ). I have tested both and they work. Now in scene 1, I have “dragged and dropped” several instances of MainButton onto my stage (from my library) . But when I call each instance’s Load_Data(str:String) function from an onLoad event in the Actionscript in the 1st cell in scene1, it doesn’t work. Can anyone tell me what event will work? I have to initialize the buttons before the user views the page.

Thank you very much!