Its falling through the floor because for whatever reason, your .cls file either isn't being detected or wasn't built with the level.
Did you add this before your main function?
string level_str = <your_levelname.WMB>; // Intro level
ifdef USE_NEWTON_GAME_DYNAMICS;
string newtonLevel_cls = <your_levelname.CLS>;
endif;
Also, is this in your main function?
// now load the level
level_load(level_str);
// initialize newton physics
ifdef USE_NEWTON_GAME_DYNAMICS;
wait(1);
dll_handle = newtonHandle;
NewtonAddMap (level_str);
endif;
If you haven't already, try adding those lines and rebuilding the level.
Hope that helps.