Where to put XPath

Once I've downloaded XPath from xfactorstudio where do I put it?

I've looked for a 'Classes' folder in the application data folder for flash but its not there. I've created one but I still can't get XPath to work.

import com.xfactorstudio.xml.xpath.*;

var myDoc = new XPathDocument();
myDoc.onLoad = function(success){
.... do something .....
}

myDoc.load(" http://<domain>/weather_proxy.asp?id=" + id); // returns xml doc


I weather_proxy script works and returns an xml document. I can parse the xml document in my code with firstChild, etc and it works, but I would like to get XPath to work from xfactorstudio. I don't think its getting imported as it never reaches the onload function.

Thanks,
Jake