Streamsheets is working with a Connector as a base and, for incoming MQTT data, with a consumer on top of it.
Go to your Stream Menu and create a Consumer based on your created Connector.
Enter a topic (you can shorten your base topic of the Connector)
Now you will see the Consumer pop up in your Inbox.
As an example, you can check out the default MQTT Streams for the Wind turbine apps.