The usual problem is the SD card. While your SD card is the right size, it might still need a "deeper" format than Windows and Mac can do. Use the SD FORMATTER tool in the attached PDF.

After you do that, I suggest you re-download the file. Oh, while you're on the website, make sure the serial number matches what's in your radio. It won't install an update for a different serial number. The serial number is on the System Settings screens - click NEXT several times to get there.

Andy