On the Diagram toolbar, click the Aggregation Link 
button.
Click on the source element.
Drag the link to the destination element.
Drop when the second element is highlighted.
Aggregation links can be drawn between all elements on the diagram, except, from or to, a collaboration element.
To create an aggregation link:
On the Diagram toolbar, click the Aggregation Link
button.
Click on the source element.
Drag the link to the destination element.
Drop when the second element is highlighted.
Related Topics:
Creating Links
Moving Links
Editing Links