There are a number of things that will trip you up. Please make sure each of these is true.
1) The card you're using is high capacity. You can tell if a card is high-capacity if there's SDHC written somewhere on it.
2) The filesystem is FAT32. How you determine this depends on your computer's operating system. If you have Windows, right click on the SD card in a file explorer, and select "Properties". It should say "File System: FAT32".
3) The firmware file is in the root directory of the file system.
Also, we do have a SanDisk 16GB card (pictured below) that doesn't work, and we don't know why. And there's a couple other SanDisk cards that customers have reported not working. We haven't gotten to the bottom of that mystery yet. So if you're having issues after following the 3 steps above, and your card is a SanDisk, please try a different card if you can. Hopefully at some point we solve the mystery.
Email firstname.lastname@example.org if you have any issues.