Change into the tab labeled Graph. Click on the button on the left labeled Node inside the box labeled Create.... Then select the created node in the canvas on the center on the screed by once left-clicking it with the mouse. Node should be highlighted. Now click on the button on the left labeled Cause inside the Create... box. Now you should see two node that are connected by an arrow. If that is not the case please check your GraphViz installation especially the dot command.