I guess I will throw in here. I was able to get this working on one of my sites. This is a page I created so I could keep up with what is going on using active desktop.
http://www.northwestdiver.com/test/activedesktest.php
I used the code pretty much just like wajones posted it. I did add a $templatesused variable for the template it uses (which I have no idea if it needs it or not. I also change include global to require global.
I put the code in a cal.php page in the forums directory. I then used an include with the full url
<?php include('http://www.domain.com/forum/cal.php');?>
It seems to work just fine.
I don't know if that helps anyone or not. Just my experience ...