Controlling Main Swf with External Swf


I had a question. My main flash file is setup with a scroll pane component. The scroll pane is loading an external swf. This external swf has a button that when pressed I want to load another external swf into the scroll pane in the main flash file.

How do I make the button functional? Is it using sprites? If so, could someone direct me to a tutorial?

Any help would be greatly appreciated. Thanks in advance.