If you've opened up a piano track, the "Software instrument" tab should be highlighted, and the "browse" tab is most likely selected. (if you don't see this, press view/hide track info in the bottom right hand corner)
1. Switch from "browse" to "edit"
2. You will see something that says "sound generator". If you have a piano sound, you will see "Piano" in a drop-down style menu. click this, then look down to where it says "Audio Unit Modules".
3. If you have installed the AU correctly, it will appear in this list. Click it.
4. It will load. You can now press the audio unit icon directly left of this menu to bring up the instrument.