If it has been at least 15 minutes since your purchase, then contact
sales@cubecraft.net with your Transaction ID, which you would have been given when you made your purchase. Make sure to include as many details as possible that may be relevant, such as the time/date of the transaction, your IGN (In-game name), and the issue that you are having.
Please note that the screenshot that you provided does
not clarify as proof of any purchase being made; you must provide your Transaction ID.
If you have any further questions, feel free to pm me on the forums :)