Okay, it must have gotten deleted somehow then. You're going to have to reinstall Address Book. Do you have access to another Mac that you could copy it from? If so, I recommend that as a first option. If not, you'll have to do it in a slightly more tricky way by using your original Mac OS X install/system disk. This is
You can do this by using a program called Pacifist, which can be downloaded from http://www.charlessoft.com/Pacifist.dmg
1) Put your Apple install CD/DVD into your computer
2) Open Pacifist and click "Open Apple Install Disks". Click "Choose" in the following dialog.
3) Once it loads (it may take awhile), navigate to:
Contents of OSInstall.mpkg/Contents of EssentialSystemSoftware/Contents of AddressBook.pkg/Applications
4) Single-click on Address Book.app and click the "Install" button
It should install back to your Applications folder, and you should be all set!
I hope this helps! Let me know if you have any more questions. Kind regards,