DLL file added to a Visual Studio (VB.net) project

I have a very simple VB.NET application which connects a database to Sage (accounts software). Since updating my computer and upgrading Visual Studio, the project has lost its reference to the DLL file which allow the SDO (Sage Data Objects) to be used in the application. The Visual Studio files and the SDO files are attached. When I try and add the [login to view URL] to the Visual Studio Project I get an error. I don't understand why. You need to resolve this and make the file part of the project so I don't have these problems in future.