Hello Alex,
Unity 5 is 64-bit while Unity 4.x and previous versions are 32-bit.
This means that you will have to make some changes to the structure of your Unity application to make sure that Unity uses the correct assembly versions.
Se this post for more information about what you need to do:
http://developer.tobii.com/community/forums/topic/eyex-framework-on-unity-5/#post-2652
Best regards
Patrik Svensson