Once you have purchased your credits use this link to redeem:
Or simply login to your Webtickets account scroll all the way to my Webtickets gift cards and select the 2nd tab redeem voucher.
Once you have redeemed your credits to your profile you can then book your tickets. On the payment page select redeem with credits.
Make sure your credits match your booking total if the booking total and credits are not the same you will need top up your credits.
Should your booking total be less than then your credits the remainder of the credits will remain on your profile to be used at a later stage.