You may have run across this issue already. When you make a new MovieClip by drawing something, selecting it, and then pressing F8 (convert to symbol), you get a dialog box that lets you choose where you want the registration point. There is a little diagram with nine squares that lets you click it to select where you want the registration point:

That's fine for when you are first creating a Movie Clip symbol. But what if you want to take an existing Movie Clip symbol in your library, and change the registration point? Specifically, suppose that you've gone through this tutorial and created the Expander class. In your fla file, you should have a Movie Clip symbol in your library called "Help Box." The following little flash application will show you how to change the registration point of this symbol after it has already been created:
(Click the little previous and next buttons in the application to go through the steps)