The type or namespace name 'ActiveDocument' could not be found (are you missing a using directive or an assembly reference?)

I'm getting the following error when adding a schema reference tothe schema library.
The type or namespace name 'ActiveDocument' could not be found (are you missing a using directive or an assembly reference?)
ActiveDocument.XMLSchemaReferences.Add _ NamespaceURI:="myCompany.schemas.com.sales_order", _ Alias:="mCsO", _ FileName:="C:\temp\sales_order.xsd", _ InstallForAllUsers:=True ...