I have my ME2 through origin now, and in it, the Cerberus network is listed as unlocked and then there are separate Normandy crash site and Zaeed DLC listed, and IIRC on my son's computer we simply clicked on them to install them...
I can't say this is exactly how it is done now, as I have had ME2 on computer since it came out...but I did add a DLC to my son's game about a week or so ago this way. The items were listed under the extra content tab in the origin pop up when you single click on a game.
As a note: using a language other than English can cause issues with DLC as most are for English only games.