Question about Web Service Path of ScriptManager

I am using VS.NET 2005. I am trying to register a web service to a scriptManager, But in my javascript I always got error saying that the web service instance was not defined. I know I probably didn't register my web service correctly. Here is my html code:

 <asp:ScriptManager ID="ScriptManager1" EnablePageMethods="true" runat="server" >
<Scripts>
<asp:ScriptReference Path="xxx.js" />
</Scripts>
<Services>
<asp:ServiceReference Path="~/ISES.WS/MainMenuOther.asmx" />
</Services>
</asp:ScriptManager>


Here's my javascript that call the  web service:

var sModNames=ISES.WS.MainMenuOther.GetExistingModelNames(vk, institutionID, outputPath,loginPath);

And here is my web service:

namespace ISES.WS
{
[WebSer ...