If you're using Tiger and Entourage 11.2 or later, you can use OS X Sync Services to keep your Entourage address book in sync with Address Book. In Entourage, go to Entourage -> Preferences -> Sync Services, then select which items you want to sync. The first sync will take a few minutes, depending on how many items that need to be synced.
There's more information on the Entourage MVP website:
Sync Services and Spotlight FAQs. (They bundled those FAQs together because we released Sync Service and Spotlight support in the same update.)
Regards,
Nadyne.