The docs at http://www.espruino.com/Troubleshooting say to download drivers from http://www.st.com/web/en/catalog/tools/PF257938
I did, but the zip file contains installer apps that fail to run in windows 10
FWIW when I plug in the Pico while holding the reset button per the instructions, I get alternating green/red for a couple seconds, then solid green and red. Device manager reports a USB serial device on com8 with "this device cannot start" status, and an "STM32 Virtual ComPort in FS Mode" under Other devices, but with "There are no compatible drivers for this device." status.