As I said before please make sure that Tobii.EyeX.Client.dll is copied to your output folder (same folder where your exe file is). Make sure that it’s also a correct version – x64 for x64 applications and x86 for x86 applications.
Visual Studio doesn’t copy native dlls automatically so you need to copy it manually or automate this process with post build actions, including this dll as content file with “copy always” flag or a build script.