Can you first verify that you have configured the iPod to sync audiobooks from your iTunes library? To do this, plug the iPod into your computer, and then when it appears in iTunes, select it from under Devices in the left hand pane. This shoud bring you to the iPod's Summary tab.
Now in the main section/window of iTunes look for the Books tab along the top and click on it. Under this tab you can configure how and what audiobooks are synced to your device. Before making any changes, first verify that the Sync Audiobooks option is ticked at the very top.
If you do make any changes from under this tab, be sure to click the Apply button in the lower right to sync the updates changes to the iPod.