Need help for my 2k12

Guys! I need help! I bought an original NBA 2k12 CD. When installing it the game didn't ask for my serial number. So it proceed installing. After installing I want to update my rosters to add the new rookies and recent trades... then when I click the ONLINE tab it says "there was a problem with your connection to the 2k sports server".

what should I do? Thanks guys!

You need to sign up for an online account if you skipped that part of the game loading. That is where you need to input your CD Key (it isnt useless) and how you can get updates.

The cd never ask me that when the game starts or before it starts.... never asked me to sign up