The purchase of credits is an automatic process, which is carried out immediately after payment. Occasionally it may take a while for the credits to be credited by the system.
Like with any purchase on the website, you will always receive an invoice on the e-mail address of the account. Please check this invoice to make sure you've actually purchased credits and not anything else (such as an additional advertisement space 'Extra advertentieruimte' or another stand-out option.)
If the credits are really not credited to your account, please contact Customer Service.