Not all copies of 007:AUF are exploitable. This post
explains how to tell if yours is.
Don't use Action Replay, use Xplorer360 beta 6. Action Replay requires you to manually add specific USB device drivers, whereas Xplorer360 beta 6 just works.
You have to plug the USB stick into your Xbox first so the Xbox can format it properly. The USB stick has to be smaller than 4GB, and have no complicated formatting (like U3) - smaller, older sticks are more likely to work.
Use the Krayzie 1.1.1 gamesave installer. Select Basic Install - UnleashX dashboard. Make sure you take a backup of your eeprom, and put it somewhere safe.