Hello I added a movieclip inside a button and I cannot access it...

I create a button, inside the button I placed a rectangle movieclip named Holder and I want to load different thing in it but I cannot access it in actionscript like: "mainButton.Holder" and I can't find it as a variable when I use a breakpoint :S

EDIT: Also this is a simpleButton, the default one that you get when you convert to a symbol

[Edited by - Tipotas688 on July 14, 2010 4:17:01 AM]