How can I import Address Book info that was stored on an external hard drive back into the Address book program on my Mac?

What format was the data stored in? If it is an Address Book Archive, exported from Address Book, you can just Import it from the File menu.

If you dragged the AddressBook folder out of ~/Library/Application Support/, you can drag it back. Just know that any existing AddressBook data will be replaced. If you need to preserve existing data, first File>Export it to an archive, and re-import it after the old data is brought back in.