How do I use a WoW Authentication Key from a Battlechest to add time to my account?

I bought a World of Warcraft Battlechest to get Burning Crusade and add 30 days to my account. I entered the Burning Crusade code without any problems, but I can't find the place to enter the XXXXXX-XXXX-XXXXXX-XXXX-XXXXXX code. It has a place for a Game Card, but not the code on the Game DVD-ROM. HELP!
How do I use a WoW Authentication Key from a Battlechest to add time to my account?
The free 30 days only works for the first time you get an account - it doesn't work if you already have one.