Te falta asignar el INDEX del array
Código actionscript:
Ver originalmyMovies = new Array();
myMovies[0] = "filenameOfFirstMovie.swf";
myMovies[1] = "filenameOfSecondMovie.swf";
index = Math.floor(Math.random() * myMovies.length);
loadMovieNum(myMovies[index], 1);
Saludos!