I recall having troubles with my Cerebus Code as well. It simply would not activate in game.
Granted, at the time it was at release day and all the servers were hammered but ...
Have you made a Bioware account?
The Cerebus Code etc does not need to be registered in game.
In fact, I had to do mine online, and then the game logged in just fine after that. (After logging into my account in game of course, and setting to auto log on)
Unfortunately I can not remember if it was under Register Games, or Redeem Promo Code.
I do recall I needed to register everything there though, then all in game worked fine.
Also after you do that, in the Registered Game Promototion will be extra DLCs and a few other goodies, all of which depends on what version and where you purchased your ME2 game. https://social.bioware.com/login.php.../user_home.php
And yea, that a pretty lame way out of helping you. ME2 specifically states Windows 7 support.
I would almost be tempted to reply with, "Then why do you advertise the game supports Windows 7?"