Click on “Dashboard” from the top menu. Then click into “Instructor-led’, which displays your scheduled classes. When you click view details on the relevant class, on the “Overview” page, the WebEx details should appear on the right-hand side with a button saying “join”. Please note this will only appear once the session has begun, you cannot join the session before the instructor has started it.
If it still not appearing, please send a screenshot of this to training.mulesoft.com/question