Did you unzip the file on the USB drive? It will not recognize the file otherwise.
If so, then it may be a naming convention - I seem to remember the issue with this, but I cannot remember the solution.
Just to make sure, a checklist:
1. Make sure you are downloading the correct BIOS file from the list (I assumed you had the same board)
2. Unzip the file into the root of a FAT32 formatted USB drive
Worst case scenario, you need a higher version than the one you have, but still not the one posted. Annoyingly, ASRock does not post all the old versions here. You might need to seek those out.