Hi!
I think you might need to use a different SDK for your project.
If you are using Unity, you should use Tobii Unity SDK http://developer.tobii.com/tobii-unity-sdk/
If you are developing for .NET (but not games), then you should use Tobii Core SDK http://developer.tobii.com/tobii-core-sdk
If you need a low level SDK for C++ then it’s better to use Tobii Stream Engine. It also has .NET bindings. http://developer.tobii.com/tobii-core-sdk
Tobii EyeX SDK is deprecated.
The error you are getting is related to the fact that your application is targeting .NET Framework 3.5, while Tobii Core SDK (and EyeX SDK as well) requires .NET Framework 4.5 and higher.
It worth mentioning that Unity uses Mono version that only implements .NET Framework 3.5. Support for .NET 4.6 in Unity 2017 is still experimental.
Tobii Unity SDK works with all Unity versions starting Unity 4.5 (on API level).