How do I use Knaken Tokens to make a purchase?

When you are logged in you can click 'buy' next to bitcoin. Next you will see a pop-up in which you fill in the amount you want to buy. Above the payment method you can click on the box titled 'use Knaken Tokens' to use your euro balance. You can't use Knaken Tokens if the purchase amount is greater than your Knaken Token balance.