Click the Association Link button 
on the diagram toolbar.
Click the class or interface source element.
Drag the link to the destination class or interface element.
Drop when the second element is highlighted.
You can draw an association link between two elements of the same type, between classes and interfaces and vice versa or between two objects.
To create an association link:
Click the Association Link button
on the diagram toolbar.
Click the class or interface source element.
Drag the link to the destination class or interface element.
Drop when the second element is highlighted.
Related Topics:
Creating Links
Moving Links
Editing Links