Your ticket will not be immediately visible in your Tomorrowland Account after you have bought it. You will receive a confirmation e-mail after your purchase. Your ticket will appear in your Tomorrowland Account shortly before the festival.
On the day of the event, you can go to the website or your Tomorrowland Account to find the link to the festival. Be sure to log in with the account you used to buy your ticket; your ticket is linked to your e-mail address.