Hi @h-scheid,
There are several SDKs out there from Tobii, the example code that you used are based on the Core SDK, but I suspect that you added the wrong nuget package for the example code. Make sure that your packages.config includes reference to Tobii.Interaction.
If you are developing for Windows 10 only, then you can also use the gaze API built into Windows. See here if https://docs.microsoft.com/en-us/windows/communitytoolkit/gaze/gazeinteractionlibrary on how to get started with those.