I am assuming that the part you want to loop is at the end of the movie. what you want to do is create a new layer call it actions. insert a blank keyframe at the end of it. click on the keyframe and then hit F9 this will bring up your actions panel. Put in this script.
in the parenthesis put the keyframe number you want flash to go back to and start playing from. This will loop back to this frame everytime it reaches the end of the movie.