Loading a dynamic location by html var

hello there,
i have some problem…
how can i load location from var in html?
this is the html code that load the flash

<div align="center" id="flashcontent" class="virtual">This content requires <a href="http://www.adobe.com/go/getflashplayer/">Adobe Flash Player 9</a> and a browser with JavaScript enabled.</div> 
<script type="text/javascript">
var so = new SWFObject("vrTapis/pano.swf", "pano", "750", "350", "9", "#FFFFFF"); 
so.addVariable("xml_file","vrTapis/tapis.xml");
so.addParam("allowFullScreen","false");
so.addParam("allowScriptAccess","sameDomain");
so.write("flashcontent");
try{        
window.document["pano"].focus();
}catch(err){
//Do nothing
}
</script>

this is the AS that load the xml file

xmlData = new XML();
xmlData.ignoreWhite = true;
xmlData.onLoad = loadXML;
xmlData.load("google_AD.xml");  

i want to get a thing like that

xmlData.load("$var/google_AD.xml"); 

how i’m doing that?
that the $var load the location from the HTML page.
thanks allot