I don't see a Use Case Diagram.
You should see the Use Case Diagram directly from the "New" button in the Explorer. If not, you would have to enable it in your workspace:
- in the Signavio Explorer click "Setup" → "Define notations/attributes"
- navigate to the tab "Modeling language" and enable the "UML Use Case Diagram"
KBA , BPI-SIG-PM-EXP , Explorer for SAP Signavio Process Manager , How To