It's "Mac", not "MAC". When you enter your serial number, it should take you to the legal release page, and then to another page where you can start the download. You can't update the firmware directly from a Mac though--the update provided is an .exe file. You either have to do the update on a Windows computer, or from a Windows system running under Parallels on your Mac. The manufacturer of the hardware that the X1 uses never provided drivers for the Mac. Doing it under Parallels does work, though--mine is set up with Windows XP for this one task only, and I've been using it for a number of years. It sounds like you're giving it an invalid serial number. Are you using the four digit number on the top of the X1, or something else? It's in the cut-out where the strap goes.
