Easy problem with Javascript that I can't fix. Please read

JavaScript
1

Hello guys, I’m developing this website:
http://www.institutoaccion.com/iaconsulting/

In the content, I’m using MooTools to accomplish the scrolling divs when clicking the subsections. This looks good and works as expected, however, I always get errors on the javascript.

This is the error:

TypeError: Result of expression '$(loopLI)' [null] is not an object.

I have found some solutions online that suggest to write the function inside the body instead of the header, but it doesn’t work for me. It seems like it a fairly easy problem to solve. Can you please guide me?

Thanks.

PD: Yes, I know the website doesn’t look nice on IE, but I give a **** about it. I want to fix important problems first.

2 
<a href="javascript:slideFolio(1);">

You cannot create links like that, since the page is rendered inoperable without javascript. Opera won’t accept it either.
Try:

<a href="#theRelevantAnchor" onclick='slideFolio(1);return false'>