I had trouble at first, too. Make sure it's formatted FAT32 and the update file is the only thing on there (or at least at the root).

Then this is where I got it to finally work. Plug in the drive (center console port) before turning on the car in any capacity. So plug it in, start the car (or just turn to "on" without starting) and wait about 30-60 seconds. It takes time to read the drive. After you've waited a minute, try the update and it should work.