Using Npgsql with Visual Studio 2008?

I need to work on a VSTO for Outlook 2007, hence I'm using VS2008 with .Net 3.5.1. The add-in will read from a PostgreSQL database, so I'm going to use Npgsql. When I compile o run the debug, I get this error:

Problem generating manifest: This assembly is built by a runtime newer than the current loaded runtime and cannot be loaded. (Exception from HRESULT: 0x8013101B)

Edit On VS2008SP1 on Windows2008R2x64, I tried Npgsql 2.2.4.3 from Github both for .Net 2.0 and .Net 3.5.1, changing the project' target framework accordingly, but the problem remains. Any suggestion please? thanks in advance!