When MQTT-Lens first opens you should see a blank screen. Click on add connection to create new connection to a broker
The only item you normally need to add is the broker name or IP address.
Click the create connection button on the button to complete the creation.
The screen should now display the publish and subscribe screen. On the left the icon should be green if the connection was established OK.
Now enter a topic (any name will do) and a test message, and then click publish to publish the message.