For me it was at the back of the box. There was a slip of paper with the maps which had the key on it.
Sorry, I don't know what to say other than try calling Bethesda or returning the game and getting a new copy :/
If its not there. return it back to the store you bought it from.
Well, back in the day, product keys would usually be found on the inside/outside of the installation disk's case, or on the game's manual. Have you checked there?
The key is on a slip of paper which is contained in the same compartment as the maps along with an ESO beta invite.
I just took a look at a couple of unboxing videos. The big book assembly came in a plastic case. The serial seems to be there.