How do I associate my Xbox 360 copy of Mass Effect 3 with my EA/Origin account?
First of all, do you already have a Bioware Social Network account? If so, then this is also your EA/Origin account. You should go to this Account Help page and check to see which of these situations apply to you.
How to tell what EA/Origin account my Xbox 360/Playstation 3 is using?
On console specifically, EA games (including Mass Effect 3) that have logins to EA servers will automatically detect an existing link between your console Gamertag/PSN name and an EA account from a previous game, and will log you in to that. This means that while you are automatically logged into an EA account playing that game (e.g. Mass Effect 3), it might not be connected to the EA/BSN account you use here - it may instead be connecting to an EA account created previously, perhaps when you first played another EA game on that console with that Gamertag/PSN name.
So in your case, if you've already connected to EA.com with another EA game on your 360, then they already know that that specific gamertag is associated with that specific EA account and you don't need to reassociate the identity.
However, you said that your console doesn't seem to be associated with your Origin account, but I would doublecheck this by logging into BSN and going to the User Entitlements page to see if your games+DLC are registered or not. If you end up having more than one BSN/EA account, then you have to figure out merging them (which is a whole other mess), which is also detailed on the Account Help page.
If you are sure your BSN/EA accounts are correctly set up, then the following is apparently the easiest way to do this from your console.
From the EA Support article here:
Your Gamertag/PSN ID and EA Account should be correctly linked when you first access an EA game online. If you are having trouble accessing features on one of our games' websites, however, you may try this alternative method to link your Gamertag with your EA Account.
- Go to the Xbox Live Marketplace or Playstation Store.
- Download a demo with online access, such as Fight Night Champion or EA MMA.
- In the Main Menu of the demo, select the "Sign into EA" option.
- If you want to link your account to an EA Classic screen name, be sure and log in with the same e-mail address and password used to access your EA Classic account.
- Accept the Terms of Service Agreement on the next page.
After following these steps your account should be successfully linked, and you should now be able to play any of your EA Games online.