@Cheyenne, I checked the file size using the command line (the correct way to do it! du update.zip) and it has 216MB, not the number from the video - a caveat on how mac calculates file sizes, if (anyone?) interested, see here. I also downloaded it again, did the steps, etc... but no luck.
@Boris, I've done that already. Thanks for the suggestion, though.
I guess I'll not try much more. Don't wanna hear from the support that I've lost the warranty because of this or that I did... Thanks a lot for the patience!!!