There are several locations to create Event Handlers in the App Designer. From the landing screen (the Application Features page), there is a left hand Applications Components panel, which includes the Case Types, the App Events, and the Global Events associated with the application.
Clicking the Expand Icon
next to the header for App Events or Global Events shows the option to create a New event handler for either an App or Global Event; clicking New opens the Procedure Designer to create a new event handler:


To edit existing event handlers, click the Edit icon next to the name of a specific Global or App Event to open the event in the Procedure Designer.
The Procedure Designer can also be opened from the Case Type Features to create Case Events. In the left hand Case Type Components panel, there is a section for Event Handlers. Clicking the Expand Icon
next to the header for Event Handlers shows the option to create a New event handler; clicking New opens the Procedure Designer to create a new event handler:

To edit existing event handlers, click the Edit icon next to the name of a specific event handler to open the event handler in the Procedure Designer.