You should be able to take the URL of the map service, paste in in a browser window and have some results returned. If you get some sort of permission error, the service has been secured and won't be able to use it without some sort of authentication. If you can't view its rest service endpoint, then you won't be able call the service in code either.
If you do see information about the service using the URL, then you have a syntax problem when constructing this layer. If you post the lines of code, it will be easier for people to see where you might have a problem.