OK, I think it works now. Thanks a lot Grant!
However, I still got a question. I downloaded Stream Engine directly from Getting Start page, then extract the whole thing into a folder in the Assets folder of Unity project. That’s it! Right now this is the location of .dll files (Assets/TargetFolder/lib/tobii) and this is the location of the header files (Assets/TargetFolder/include/tobii). Is this the right location for the header files?
Also, I think you really should mention files in “binding” folder in the setting guide in Getting Start section. Because you only mention files in “include” and “lib” folder, I didn’t have any idea what files in the “binding” folder does and didn’t think I would need to paste those into my project at all. Like I said, pasting those files in Unity project make it work instantly.