ArcGIS Explorer is a client application of a machine running ArcGIS Server. This machine is considered your Home Server and hosts the services and resources you'll be using. You may hear it referred to as a server object manager (SOM) machine. You or your administrator must establish which machine will be your home server. By default, the Home Server for ArcGIS Explorer is one of ESRI's servers. You or your organization are encouraged to use this server; however if you or your organization have an ArcGIS Server license and have set up the server, you should make it your Home Server. To change this setting, use the dialog box that appears when you start ArcGIS Explorer for the first time or by clicking File > Set Home Server. The dialog box looks like this
Specify a name of a Home Server in the form: http://myserver_name/arcgis where myserver_name is the name of the SOM (Server Object Manager) machine and arcgis is the folder containing the ArcGIS Server application- and service-specific items. The application enables the Test button if there is a need to verify that the path is accurate; a valid path does not require verification.
Once you've set the Home Server, click OK. ArcGIS Explorer will restart the application, displaying the default map of the Home Server or a map you specified in the Application > Startup panel of the Options dialog box.