Some folks have had problems redeeming their Xbox Live codes for the Gray Fox skin that came as a bonus with pre-ordered copies of Metal Gear Rising. But Konami says they have fixed the problem, and so you should be able to plug your codes into Xbox Live now.
What happened was Microsoft forgot to active whatever system allows the codes to be used, and so gamers just had to wait a little bit.
I don’t know why you would want to play with a Gray Fox skin anyway when you get to wear a poncho and sombrero. That’s the real way to go.