When you buy an audiobook from iTunes where do you open it to hear it?

If you download or sync an audiobook to the iPad then you can listen to it in the Music app - when their audiobooks on the iPad you should get an Audiobooks option under the More button at the bottom of the Music app.

Note that audiobooks are currently a one-time only download from the store, so if you buy any directly on the iPad you may want to connect the iPad to your computer's iTunes and do File > Transfer Purchases (File > Devices > Transfer Purchases on iTunes 11) so that you can take a backup copy.