I have a MapServer Service published in ArcGis For Server. I want to add a new Layer to this Service. how can I do it? any help is appreciated
You'll need to republish the service using ArcGIS for Desktop if you want to add new layers to an existing service. For details, see the documentation: Overwriting a service in ArcGIS for Desktop
Another option is to publish a second service with the new layer and then use both services in your application.