Thanks TheArmKing! I have tried this on SpookizLink game but the mod menu isn't working. Even the one created by PeekABoo doesn't work too. I had success getting the mod menu to show using AndnixSH's mod menu (available on his website) though.
My guess is the UnityEngine.dll packed doesn't have the full functions of GUI like most game does. In fact to make AndnixSH's mod work, I had to remove all the "background" property in his code, leading all the hacks to be overlay-ed onto one another, in a single line.
Any way to overcome the problem or "force" the full classes/functions of UnityEngine.dll into the existing one?