Just click "Play Now" button on home page, A new window will open. Go to "Tourneys & Freerolls" tab and select the "Tournament that you want to participate" and click on register button on right side. You will get registered and once at the start of the tournament you will be able to play
For example "Sat 50k GTD Tourney" will start exactly at 8:30PM IST
So make sure that you are logged in and the game lobby is open before 8:30 PM.
The tournament table will pop up exactly at 8:30 PM
You will be able to play after that.