Can't get script to work

Sorry for the (probably) dumb question. I've downloaded a PHP script and have got most of it working for a banner rotator. However, I now need to pu the following bit of script into my webpage:

<?php
include_once("/PATH_TO_SCRIPT/ad_rotator.php");
?>

I can't work out what to put in place of PATH_TO_SCRIPT

My webpage resides in a folder called Actinic at www.mydomain.net/Actinic and my php script resides in a folder called ADV at www.mydomain.net/ADV

Would appreciate any help please!!!

On your webpage at the very top add this
<?php
include_once($_SERVER['DOCUMENT_ROOT']."/ADV/ad_rotator.php");
?>