I'm a bit of an XSL Noob so bare with me here...I have a JavaScript function:

<script type="text/javascript">
        <!--

        d = new dTree('d');

        d.add(0,-1,'My example tree');
        d.add(1,0,'Node 1','example01.html');
        d.add(2,0,'Node 2','example01.html');
        d.add(3,1,'Node 1.1','example01.html');
        d.add(4,0,'Node 3','example01.html');
        d.add(5,3,'Node 1.1.1','example01.html');
        d.add(6,5,'Node 1.1.1.1','example01.html');
        d.add(7,0,'Node 4','example01.html');
        d.add(8,1,'Node 1.2','example01.html');
        d.add(9,0,'My Pictures','example01.html','Pictures I\'ve taken over the years','','','img/imgfolder.gif');
        d.add(10,9,'The trip to Iceland','example01.html','Pictures of Gullfoss and Geysir');
        d.add(11,9,'Mom\'s birthday','example01.html');
        d.add(12,0,'Recycle Bin','example01.html','','','img/trash.gif');

        document.write(d);

        //-->
    </script>

I need to be able to use the function in my XSL using the add function.

Some sample xml i am looking to transform would be similar to
<Policy>
     <id>0</id>
     <pid>5</pid>
     <name>Sample Name</name>
     <url>myurl.html</url>
</Policy>

so ideally i would write the function as:
d.add(0,5,'Sample Name','myurl.html');

which is:
d.add(id,pid,name,url);

how do i go about doing this?

Your Javascript function looks like something that lives in an HTML page. XSLT is generally used to generate an HTML page. Clearly you can't call a function in the page that you are generating, because it's not available until you have finished. So I don't really understand what you are trying to achieve.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference