Mass Effect 2 Cerberus Network

These Mass Effect 2 add-ons not available in Xbox Live service,

only available in Cerberus Network

(which can be found in Mass Effect 2 main menu / Extras submenu, and requires Mass Effect 2 activation code).

you need to redeem that code in game through the Cerberus Network

this guide might help

Update regarding inclusion of DLC.

What DLC is included in the Mass Effect Trilogy?

• On PC, Mass Effect will include Bring Down the Sky and Pinnacle Station on disk. For Mass Effect 2, Cerberus Network will be included which features Zaeed – The Price of Revenge, The Firewalker Pack, Cerberus Assault Gear, Arc Projector heavy weapon, and
  Normandy Crash site mission. For Mass Effect 3, Online Pass will be included granting players access to co-op multiplayer
  
  

• On Xbox 360, Bring Down the Sky and Pinnacle Station are not included with Mass Effect, however they are available as stand-alone downloads through Xbox LIVE. For Mass Effect 2, Cerberus Network will be included and Online Pass will be included for Mass
  Effect 3.
  
  

• Information on PlayStation 3 DLC will be available soon.
  
  

Source

I do wonder how long they are going to keep these operations going now that Mass Effect 3 has ended support.

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The Cerberus Network for Mass Effect 2 lasted a fair while, so hopefully these will too.

Mass Effect Genesis not not part of the Cerberus Network on the 360, only the PS3.