Flash - Changing SelectedIndex From AS File?

I have a simple button function that changes the selectedIndex on a view stack without using the menu bar component. I call the function from the click property on the button and pass an int as the index I want to jump to. When I had the code on the application MXML file everything worked fine. After moving my actionscript to a separate file, I get access of undefined property errors:

protected function changeView(index:int):void
{
myViewStack.selectedIndex = index;
}

How do I get the .as file to recognize the myViewStack component? Do I need to reference Main.MXML somewhere in the .as file? Thanks.

Here is the rest of my code:

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"

[code]....

And finally the .as files with the function in question:

import mx.containers.ViewStack;
protected function changeView(index:int, myViewStack:ViewStack):void
{
myViewStack.selectedIndex = index;
}