> I have been trying "_root.grid.gotoAndPlay('play');"
> others but no joy.
Since you used loadMovieNum() to load grid.swf into level 1,
to reference _level1 in order to reach that SWF. (The _root
to the currently active level, which is _level0 from the main
_level1.gotoAndPlay("play"); should do it. There is no "grid"
reference to speak of in this context, because loadMovieNum()
you to give instance names.
You might want to experiment with loadMovie(), or even
MovieClipLoader class, since both of those allow you to route
SWF into a movie clip symbol with an instance name. (Provide
via the Property inspector.) MovieClipLoader even provides
events to let
you know when the content has loaded.