Call ASP.NET 3.0 Web Service from JavaScript

Hi,
  I'm having problems calling my web service from JavaScript.  (I'm using .net framework 3.0.)  I'm trying to call a simple HelloWorld function.  What am I doing wrong?  I know the web service is working correctly, but can't figure out how to properly call it.

 
             JavaScript:
            function InvokeWebMethod() {
                alert('test');
                ItemService.HelloWorld(cb);
            } 
            function cb(result) {
            &nb ...