Another potential problem is that you're inserting it in the design frame, it needs to be inserted in the Code part at the top of the picture.
[IMAGE: http://i20.tinypic.com/4h62qf.png]
I don't think it'd be able to figure out that
<script type="text/javascript"... is the same as
<script type="text/javascript"...
This could also be why if searching for
<script nothing's coming up. It's there, just not in the right place.
[IMAGE: http://i24.tinypic.com/2rpslfm.png]
I think this is what you want:
http://www.4filehosting.com/file/77325/index-html.html
Basically, either download the file I edited (hopefully correctly!), or instead, as opposed to putting the advert code where you are now, look in the HTML code frame (at the top) for where it says "<body" and a load more text, and the copyright notices in your HTML file, and paste it just there. Or you could put it before "</body>" at the bottom if you want the advert to be at the bottom of the page.
Paul.