Building an ArcGIS Explorer add-in project in Visual Studio can result in one or more validation errors in the Visual Studio Error List window with the following message:
"Validation failure: Could not find schema information for the element 'http://www.esri.com/schemas/ArcGISExplorer/1500/Addins:ExplorerAddIns'."
From version 1500 onwards, the AddIns.xml file created by the ArcGIS Explorer Tools for Visual Studio contains a new XML namespace attribute. This namespace is not applicable to previous releases and causes a build error if the add-in project created with ArcGIS Explorer 1500 is built on a machine on which ArcGIS Explorer 900 or 1200 is installed.
If new build errors are found, such as 'The type or namespace name 'CheckBox' does not exist in the namespace 'ESRI.ArcGISExplorer.Application' are you missing an assembly reference?)', the add-in may be using functionality that was introduced in a later version of ArcGIS Explorer than the version currently installed. Resolve any issues by using alternative code that is supported by the ArcGIS Explorer version currently installed. Refer to the ArcGIS Explorer Developer Help for more information on new API types and members added at version 1500. The Developer Help is installed as part of the ArcGIS Explorer SDK for the Microsoft .NET Framework and is also available online from the Esri Web site at ArcGIS Explorer Online Help.