If you have bought your ticket via your Tomorrowland Account, the link to enter the festival will be visible in the account you used to buy your ticket.
If you have bought your ticket via the official Tomorrowland Around The World ticket shop without an account, you need to create an account with the email address you used to buy your ticket. The link to enter the festival will be visible in this account.
On the day of the event, you will be able to enter the festival via the website or via your Tomorrowland Account. Be sure to log on to the correct Tomorrowland Account; your ticket is linked to your email address.