Start by choosing Store> Check For Available Downloads in iTunes. It should see the audiobooks, which should appear in the Books dropdown menu. If that works, you should then connect your iPad or iPhone to your computer. Open the device, and click on "Books". Then select "Sync Audiobooks". If this completes. and the audiobooks sync to your device, they should then be in the Music app, under More (...) > Audiobooks.
You may need to either close and re-open the Music app, or possibly restart your iPhone or iPad to get them to show up.
Do you have iTunes Match, by chance?