Before you try to join a class, make sure you are logged in on the site.
It is possible, that you already received the link of the Virtual Classroom via e-mail, but the class hasn't started yet. When the class has started, you should get another e-mail "-Name of the class- has been started, join now!" subject and the link of the Virtual Classroom. It means that the Virtual Classroom is open, you can join it by clicking the link.