To complete the shape tween and make the fireworks burst shrink to nothing again, all you have to do is copy the first framewhere you already scaled the shape downto the last frame, frame 24. Then repeat the process in the last step, and select frames 13-24 (dont select frame 12!) and apply a shape tween to them in the same way. When you drag along the numbers above the timeline you should be able to watch your animation in full as the burst grows, then shrinks again. Dont forget to save your work when youre done, and then exit out of the symbols editing stage by clicking on the blue arrow in the upper left hand corner of the stage.
Movie clips wont play in your main working area until youve actually exported the final, but there are playback controls in the library to view the actual symbols animation. You can preview the fireworks burst by going to File->Preview and watching how it looks with your entire scene. It looks a little silly now, but itll look better by the time were done.