Hi Rui,
Maybe it too late, but it's easy step to create mqtt connect.
After compiled and installed the MQTT Input / Ouput Transports, you just go to GeoEvent manager and followed steps.
1. Go to Site -> Connectors -> Create Connector
2. Adapter: Generic-JSON
Transport: MqttInboundTransport
3. Setup advanced properties
4. Click create
5. Enjoy it
Saga