Did you unzip the download BIOS file, placing all of it's contents on the USB stick? Try placing the files in the root of the USB stick, not in a folder. Also, within BIOS make sure Legacy USB Support is set to Enabled. Your USB stick should already be plugged into your computer when you reboot to attempt the flash. If it doesn't work, try another USB slot.
I tried unzipping and placing all the files there too... nothing worked... legacy support is enabled, and it just doesn't find the files on the usb... and on a couple of usb slots, the drive doesn't even work... I found one that does, but it just says either previous folder, or media files... media files just returns me to partition 0.