Click the Communicates Link button 
on the diagram toolbar.
Click the source element.
Drag the link to the destination element.
Drop when the second element is highlighted.
To create a communicates link between an actor and a use case:
Click the Communicates Link button
on the diagram toolbar.
Click the source element.
Drag the link to the destination element.
Drop when the second element is highlighted.
NB: A communicates link applies only between an actor and a use case.
Related Topics:
Creating Links
Moving Links
Editing Links