There are two other ways you can create an explicit link between a use case and an actor, or between two use cases:
When you create an explicit relationship in the Relationships Browser, it is listed as an existing relationship in the Use Case Diagrammer. However, it will still not appear in the diagram unless you make it visible using Hide/Show Relationships on the pop-up menu.
Association or
Extension (depending upon the kind of link) to connect
them. Creating a visible association in a use case diagram
automatically creates an association between the underlying model
elements.