It is the owlcms-firmata software that talks MQTT, and not your device by itself (or the blue-owl software if you bought blue-owl devices).
If you use the latest control-panel, the owlcms-firmata software is installed under Arduino Devices.
You need to run that software on the laptop where your refereeing devices are connected, and tell it where owlcms firmata is.
Owlcms does not "find" MQTT devices, owlcms-firmata is told where owlcms is.