Specify an instance of SQL Server Analysis Services for Excel Services

To use data model functionality in Excel Services, an instance of Analysis Services must be specified for the Excel Services service application. The instance is specified in the Server Name box (which is located on the Data Model settings page for Excel Services service application management in Central Administration) and uses the form SERVERNAME\INSTANCE.

For more detailed information, see TechNet Article: Manage Excel Services data model settings (SharePoint Server 2013).