You'll need to unlock the bootloader and proceed from there. You can do it the Pudding way (messy), but I also found
this over in the Atrix section, which looks far simpler. It will take you back to 2.3.4, but if your bootloader isn't unlocked then you'll have to do that anyway
If your bootloader is already unlocked, use the new version of SuperOneClick with the Motorola exploit (link in my sig). It works a treat
Good luck!