Well, once you finish drawing the garbage matte, and want to start animating it, you have to switch from the Draw tab to the Animate tab in the toolbox. You might find this video tutorial
of use in learning how to work with garbage mattes. Also, searching the manual for "garbage mattes" will give you a few pages that explain every part of the tool in detail. Also, you might benefit from this knowledgebase article
which discusses the differences between garbage mattes and masks, and when is the proper time to use each.