I know that others have mentioned this one before, including Stefan, but if you find that your tree-view control isn't loading, it's likely that your website isn't running under the wwwroot default folder. This means that you probably don't have the webctrl_client folder under your web root folder. Copy it over from wwwroot and that should sort you out.
You might also need aspnet_client if you're using anything that utilises client-side ASP.NET generated scripting, such as validators.