I take no credit for this, it is just a repost which will hopefully help some of you out there who dont want to spend 480 MS points on something which should be free anyway. Thanks to Thelorn on x360a for the repost of Oreopatch's post.
"The 4 universal codes are as follows.
You will need to make 4 different Dr.Pepper accounts.
Try using Internet Explorer if you have trouble with others.
I have edited the original post to add a screenshot step by step guide made by forum member VinasErgoth so for that I thank you.
I DO NOT take credit for this find. Credit is due to poster Oreopatch from the following Bad Company 2 Official forum. http://forum.ea.com/eaforum/posts/list/505226.page
Go to http://www.drpepper.com.
Scroll down until you see the EA Promotion.
Insert one of four universal codes into the box, and click the arrow next to it. When it asks what size and where you bought it, just fill in anything. It doesn't matter.
Scroll down to the registration bit, and fill in any old crap.
See the big pink "CLAIM NOW" bit? Yeah, right click that and copy the link address. For this tutorial I used the latest version of Firefox.
Paste the link into your address bar, and the very end should look something like the one in the picture. Just replace the 3 on the end with a 4 (if it's already a 4, replace with a 3) and press enter.
Click continue on the next screen
Choose Battlefield Bad Company 2 as your game across the top. It might not have a picture. Then hit continue.
For the sake of the tutorial, I've had to choose something else as my promotional because I've already redeemed the 4 kits. It's exactly the same though. Choose a kit and hit continue.
Once you get this screen, you're done. Just hit the little X that I've highlighted.
You're done! This is the confirmation screen that loads once you hit the X on the previous page.
Hope this helps some of you."
Original post contains screenshost for those who struggle with it: http://www.xbox360achievements.org/forum/showthread.php?t=224040
Worked for me, but theres been mixed experiences, I was also away from my xbox for 3 days before turning it on to find the unlock, so I cannot confirm if it is instant either.
Always worth a try though, and if it doesnt work, use the EA support chat, and say youre from the USA, they will activate the unlocks on your account :)