And @Daelmo
How to see if the dll is correct version between Northwind.DAL and GUI?
The SD.LLBLGen.Pro.ORMSupportClasses.NET20.dll is suppossed to be referenced in both the of the projects, the gneerated DAL and your GUI application.
You can open the properties of the reference in each project and check the version number, or make sure they are pointing to the same path.
(Edit)
Please try to rebuild the entire solution. (DAL + GUI).