Control.Scrollbar IE null error

Hi,

I am using your Control.Scrollbar in this page (still in development): http://www.latinoimmigrantstories.org/stories.php

IE 7 and 8 returns this error on page load:
Error: 'null' is null or not an object

and it referencing this:
$('scroll_down_50').observe('click',function(event){

    scrollbar.scrollBy(-50);
    event.stop();
});

Is there another step in my file that I'm missing?

Thank you for the wonderful scrollbar!

Lisa