Solved this myself.
Some better instructions:
1. Start with the board unplugged
2. Copy dfu-util.exe to the folder on your PC that has the uncompressed image files
3. Run flashall.bat from a Windows command prompt in that folder
4. When it says "plug and reboot", plug in the USB cable(s). You may need to turn on your board.
5. The board should be detected and you should see lots of activity. Follow any instructions.
What did NOT work for me: the standard instructions where you copy the image files to the root of the Edison drive and type "reboot ota" in the Edison console.