The Send Message Response Workflow Activity sends a message in response to a Received Message (received through a Receive Message Workflow Activity).  This activity is towards the top of the Activity Palette in the Workflow Designer.

To use this activity in a workflow, you must set the IncomingMessage and ResponseMessage parameters by selecting from the drop-down list of configured messages.  On execution, these are respectively the names of the message to which you are replying, and the response message you are going to send.