I think I understand what is happening. The compiler compiles all source files it finds in the folder. So when compiling the sketch itself everything goes fine I think. When the compiler comes across the MyTinySensors.cpp, it tries to compile that file also but can't find the define MY_NODE_ID. So I think you should move these files to subfolders (like done in the MySensors structure), so they won't be compiled individually anymore. Sounds plausible right?