To access ArcGIS Server services in ArcGIS Explorer Online, the ArcGIS Services Directory must be set up and functioning. To verify this, navigate to http://<host>/arcgis/rest/services in a browser. The Services Directory main page should display.
If an ArcGIS Server map service is not publicly accessible, the host server must have a clientaccesspolicy.xml file in its Web server host directory to allow ArcGIS Explorer Online to access it. For security reasons, a Web browser is not allowed to access data that resides outside the exact Web domain where a Silverlight application (in this case ArcGIS Explorer Online) originated. However, permission to access data across domains can be granted by a server by having a small clientaccesspolicy.xml file on the server. For more information about using a clientaccesspolicy.xml file, please refer to the topic in the Related Information section, Network Security Access Restrictions in Silverlight.
When specifying the URL of an internal server as the source for adding map service content, include the domain name in the server URL. For example:
http://myserver.esri.com/arcgis/rest/services/USA/MapServer (instead of http://myserver/arcgis/rest/services/USA/MapServer.)