Hi Veronika,
is it the out-of-the-box SDK package that won’t build? Or did you create a WPF project of your own?
The Tobii.EyeX.Client.Net20.dll is a .NET wrapper for the EyeX C API and is used by the EyeXFramework assembly. It is usually copied to the EyeXFramework output directory when that assembly is built. Can you please verify that it’s there? (I would be surprised if it wasn’t as the WinForms samples work.)
One explanation that I can think of is that the WPF project and the EyeXFramework project are built for different platforms. Open the Build/Configuration manager dialog in Visual Studio and verify that everything is built for x86. For some reason, Visual Studio insists on changing the build platform to Any CPU every now and then. And then it won’t work.