Thanks again, Neil.
When I first typed in using ESRI.ArcGIS.Carto, I got the following error message: The type or namespace 'Carto' does not exist in the namespace 'ESRI.ArcGIS'. Then within the Solution Explorer I right-clicked References >> Add Reference... and in the .NET tab I selected ESRI.ArcGIS.Carto. Then no more error messages.
There are quite a few ESRI references in the list of references under .NET. Is there some source which shows which specific references are required for specific interfaces which a user wants to use?