Click on Dashboard from the top menu of your Training Account. Select Instructor-led, which displays your scheduled Instructor-led courses. Select View details on the specific class, then select the Overview page. The WebEx details should appear on the right-hand side, and a button will appear with the word Join. Please note Join will only appear once the session has begun, you cannot join the session before the instructor has started the course.
If the Join button is still not appearing, please send a screenshot of this to training.mulesoft.com/question along with a description of your issue.