AFAIK "static std::string LevelDataString" in the class definition isn't enough. It's only in a header file, but it doesn't have a place to stay, in your current code, it does not exist.
You have to put "std::string Level::LevelDataString" somewhere in a .cpp file. It could also be because you're using a too old version of the SDK, without support for C++ as it was first added by tangrs somewhere around r865.