In order for the Transfabulator to grant you an item, you must have the same type of item that it will be giving out. For example, if the pool of items that the machine is giving out contains a hat and trousers, your Neopet must be wearing one of those items upon entering. Play around with items in your customisation application and keep trying the Transfabulator. Once you've redeemed a Key at the door, you will have access to the Transfabulator until you press the lever and receive a new item. So, if you have to leave the page without using the machine, you can come back without having to redeem another Key.