WOW Gold there should be two authentication keys. For the Wrath of the Lich King Expansion set

And for the wow Battle Chest,Cheapest Reliable Gold WOW, there should be two authentication keys. For the Wrath of the Lich King Expansion set

or on the front of the paper sleeve of the DVD if you have a Standard Edition. f you receive a message "your authentication key has already been claimed"

your authentication key should be located on the front of your manual if you have a collector's edition