When a Virtual Show becomes available, the area in the middle of the Lobby will be marked with a "Live Now!" sign.
Move your Avatar to this area, and when a pop-up appears, tap "Enter."
You can also access a Virtual Show by tapping "Enter" on the top right of the Lobby screen.
Note that once a Virtual Show becomes available, the show itself won't start immediately. You can check when it will begin from within the Lobby.