Ok so I got Mass Effect brand new and have the code for Cerberus. I have not activated it yet due to the directions not being very clear. Can I just activate it in-game on my 360 like any other DLC code or do I need to do something else? PLEASE HELP.

It's all in game. It's totally separate from the Microsoft code system. When you boot up Mass Effect 2, it will ask you if you want to connect to Cerberus Network or something like that. It's all reasonably straightforward from there.

So I don't have to do anything with EA accounts or linking anything?

I don't remember the directions very clearly, but if it's like Dragon Age II, then you might have to enter that specific code for your Bioware account. Does it look like a regular Microsoft code? Bioware codes are shorter if I remember correctly.

It's been a long time since I activated it, but isn't there just a "redeem code" menu option in there somewhere?  You do need to have an EA Online account before you can redeem it, but you should be able to create that in-game, if you don't already have one.

Thanks guys. I just went with my gut and activated it in-game. Worked without any problems, and all the DLC has been added.  
Just needed to get clarification since there have been a lot of issues with this service.