Could you possibly give more details...? When you were trying to fix it, you may have inadvertently messed something up.
Was it working before you downloaded these meshes? Where did you put them?
It sounds like a error in the game to me, where it couldn't find needed files. (The "FileNotFoundExcpetion")
Also, does FH only act up when your trying to edit a map? Or when you actually startup FH?
Also, when does the error occur?
Just need some clarification before I can help in any real detail. But, I hope I can. ^^
~Kekitan