You must choose "Legacy of the Void" in your options (Gameplay - Expansion Level).
Sign in to a tournament using "Sign up":
(Link to screenshot)
After "Sign Up" you will see something like this:
(Link to screenshot)
A "2-minute-warning" will appear and you have to click "Ready" to participate.
(Link to screenshot)
Sometimes at first the tournament will start with a group stage (e. g. Friday to Sunday).
You can make "Map Vetoes" if you like.
(Link to screenshot)
If you loose it will look like this:
(Link to screenshot)
If you win it will look like this:
(Link to screenshot)
A 5-Minute-Video in high speed:
GG!