Navigate to the Management Tab of your warzone. You'll see a list of all players and in the table there's a link that says, "Unpaid". If you click that link it will switch to "Paid".
Important note: iToysoldiers doesn't handle the admission fee payments and the Paid/Unpaid flag is only and indicator for your records. It has no impact on any mechanisms of the leagues, tournaments, or events on iToysoldiers.