Dear all, thank you for your suggestions.
This is what I have done:
1) Hostname: localhost
2) UserID: Tomas
3) Port: 1883
4) Password: –blank–
I actually had to restart Rocview/Rocserver for these changes to take effect. Once I have done so, I saw a lot of MQTT traffic in the CMD window running MQTT (mosquitto -c mosquitto.conf -v).
Then I moved the slider on the locomotive control – and bam, the PU bogie connected to the hub started spinning away. Love it! It is a first step toward automating my mini layout.