Use the in-game store or this link: http://pixelminegames.com/Shop/

Then:
  • choose what you want to buy and click "view cart"
  • click "proceed to checkout"
  • enter all your info including "ACCOUNT NAME" and "place order"
  • You're done
Note you will need a Paypal account. Token transfer must be manually done by support; it doesn't happen automatically so there may be a little wait of a day.