Click the Uses Link button 
on the diagram toolbar.
Click the source element.
Drag the link to the destination element.
Drop when the destination element is highlighted.
A uses link can be drawn from interaction occurrences to lifelines or between two lifelines. The "uses" stereotype displays above the link on the diagram.
To create a uses link:
Click the Uses Link button
on the diagram toolbar.
Click the source element.
Drag the link to the destination element.
Drop when the destination element is highlighted.
Related Topics:
Creating Links
Moving Links
Editing Links