How to get the DOM tree from a document?

[Daniel Kirsch]

Hi,
I want to load a XML document in my XUL application and access the 
elements with the DOM.
May someone give me a hint how to do this?

So far I am able to get the file using nsIFilePicker, load the document 
and view it's plain content. I tried the following with no result:

var treeDOM = document.implementation.createDocument("","tree",null);
treeDOM.load("chrome://ida/content/course.xml", "text/xml");
However my treeDOM objects doesn't have any content.

Thanks for any help
Daniel