Do not know why, but it seems that Unity has some problems with the 64 bit version (or i forgot to set some settings ? anyway ).
If someone has the same problems:
– use the 32 bit dlls
– copy TobiiGazeCore32.dll into the project root folder
– copy Tobii.Gaze.Core.Net.dll into assets/plugins folder
That worked for me.
Edit: Oh and make sure there are no _just_in_case_ dlls of 64 version in your project.