In iTunes, with the iPad connected, there should be a tab titled "Books." That tab will have two check able categories, "Sync Books" and "Sync Audiobooks."
If you select/check "Sync Audiobooks" and then check "Selected audiobooks," all your audiobooks should show for you to pick them. If you want to sync all audiobooks in iTunes over to the iPad, you can select the "All audiobooks" line. Note that if you do that, you won't see the list of audiobooks.
Hope this helps/clarifies.
Marilyn