Click the Extends Link button 
on the diagram toolbar.
Click on the use case source element.
Drag the link to the destination use case element.
Drop when the second element is highlighted.
To create an extends link between two use case elements:
Click the Extends Link button
on the diagram toolbar.
Click on the use case source element.
Drag the link to the destination use case element.
Drop when the second element is highlighted.
NB: An extends relationship link applies only to use case elements.
Related Topics:
Creating Links
Moving Links
Editing Links