I helped a friend set up an ATT Uverse account, and the code was on the black box. The box in this case was a combination modem and router. Look there. If it's not there, try the last 4 digits of your phone no. If that doesn't get you working, keep calling until you get someone who can help you.
Good luck.
att uverse internet it says i need a four digit passcode but the sales rep never asked me to make one and I've called their support and they claim not to know or have even heard of a passcode being required. What should i do?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic