Yesterday, I used this manual to set up an RPi3 and it worked well.
The Edge Client is now online on MQTT connected to IoT Open.
Since I'm not that Linux guy a used WinSCP to edit the iotopen.json and I don't know how to use "sudo" in WinSCP so I first created the file from the terminal using "touch"
admin@IoTOpenRPi1:/etc/iot-open $ sudo touch iotopen.json
and to enable my WinSCP user editing the file, I changed the user privileges a bit
admin@IoTOpenRPi1:/etc/iot-open $ sudo chmod 777 iotopen.json
admin@IoTOpenRPi1:/etc/iot-open $ ls -la
total 24
drwxr-xr-x 2 root root 4096 Apr 24 17:23 .
drwxr-xr-x 134 root root 12288 Apr 24 17:01 ..
-rwxrwxrwx 1 root root 316 Apr 24 17:26 iotopen.json
-rw-r--r-- 1 root root 518 Mar 4 16:34 mosquitto.tpl