Go here
https://apps.facebook.com/xboxmasseffect/uk
Launch the app, fill in your Gamertag. And just follop the steps. personally i didn't even know what to do, i then saw something called 'launch decoder' and it quit the whole thing for me. I then saw a button called 'view my codes' and whatta ya know...the ME3 demo code right there.
It worked for several friends of mine, my bro and many more. So this can't be coincedence anymore.