Hi Ashkan,
What version of Unity are you using? We have seen a problem when using Unity 5 on a 64bit system, and then build a 32 bit standalone – for some reason the 64 bit version of the Tobii EyeX client dll gets copied to the output folder instead of the 32 bit version. In this situation you have to copy the client dll with the correct bitness to the target folder manually. We have not found a way to make this work right automatically with the build scripts included in the EyeX SDK package – it seems Unity is doing something magic behind the scenes here – redirecting or using a copy of the dll used in the editor – we don’t know.
EDIT 21 Dec 2015: This issue was tracked down to a bug in our editor script. It has been resolved in EyeX SDK for Unity 1.6.