I managed to get mine updated too. Here's what I had to do after I installed the USB drivers & the Line 6 Updater:
1) download the HDX firmware file into the local Line6 updater location: C:\program files (x86)\Line 6\Line 6 Updater
2) turn my computer speakers on so I could hear the actual connection over USB being made. I was expecting to see something change on the HX Stomp display and that doesn't happen.
3) power the Stomp on holding down the right page arrow button. You will here the sound your computer makes when it finds new hardware.
3) run the Line 6 updater and tell it to run in offline mode. If you have a USB connection to the Stomp it will ask you if you are updating an HX Effects or an HX Stomp
4) pick the HX Stomp
5) click the browse button and you should see the hdx file, run it and enjoy.