You are a victim of non-compliant browsers from what it looks like to me. You manipulating everything the Microsoft way. Netscape references everything in a completely different way. You should go to a site like: www.siteexperts.comwww.htmlguru.com (particularly his wandering eye tutorial)
as all the sites have good tutorials on manipulating layers in both browsers and degrades well on older browsers. The other thing you should do is move your script up into the HEAD section of your page or into its own file and link it in with a script tag in the HEAD section of your site. The only thing your doing now is breaking the W3C standard document definations. The script won't run until everything is loaded anyways because its triggered by the body's onload event.
As it is written now you have cut out users of navigator plus anyone using a browser earlier than version 4 from seeing the demo. You should add a browser sniffer to the beginning of the script to redirect users with older software directly to your main page. As to why Mac users with IE4.1 and higher can't see it, I don't have any clues unless they did not install the Microsoft Jscript Engine for some reason.
There's a useful utility called browserola (alas, I don't have the site url), that emulates various browsers. This is especially useful if you want to test your site over several versions of a browser.
Also, when making a site, I tend to make it work in IE, and then see if it can work in NS, and optimize it accordingly. Hopefully XML will stop all this work
With some things you will have to reside to two or more site versions. That's the ban of web developing. But I wouldn't trade it for anything, except maybe a job in a comic store!