Is your app in the root? If your app was http://localhost/myapp/ then the url you'd have to request would be http://localhost/myapp/ScriptResource.axd not just localhost. The best bet is to paste the url directly from the view source of your page, including the long querystring it contains.

You need the ScriptResource entry that contains the fully qualified name, although it would probably work either way, as long as the extensions dll is also mentioned in the <assemblies> section.