I don't want to fix this problem directly, But I want to suggest you to use frameworks and APIs that would make AJAX easier for you; Your high level requirement is that you need to call a server side method from the client side. So don't confuse yourself with low level APIs when there are some free frameworks for AJAX.
I have used DWR before this(1 year ago) and I was happy with that.
I hope those comments fix your problem indireclty