Use this command to generate a key:
cat /dev/urandom | tr -dc '0-9A-F' | fold -w 32 | head -n 1 | sed -e 's/\(..\)/0x\1, /g'
And add it to your configuration.yaml example:
zwave:
usb_path: /dev/ttyACM0
network_key: <ouput from the above command>