Use the add method for related fields:
# using Model.object.create is a shortcut to instantiating, then calling save()
myMoveInfo = Movie_Info.objects.create(title="foo", overview='bar')
myMovieGenre = Movie_Info_genre.objects.create(genre="horror")
myMovieInfo.genres.add(myMoveGenre)
Unlike modifying other fields, both models must exist in the database prior to doing this, so you must call save before adding the many-to-many relationship. Since add immediately affects the database, you do not need to save afterwards.