If you're going to test the service on the SMP, you have to remote/get on to the SMP itself to test the method. Browse to it via IIS once you're on the machine. You might not able to invoke it then if the method has complex inputs.
And if you are getting a message similar to "collection guid is not valid", you ARE hitting the web service, and it doesn't like the GUID you've provided for whatever reason. I'd verify it's valid.