The easiest way to pay your Microlink bill (or topup your PAYG account) is to log into the Microlink accounts portal (the login form on the right of this page) and click on the link in the right-hand links area that says 'Pay Bills'.
(If you don't know your account portal credentials, you can get them here.)
This system accepts Visa and Mastercard, including Barclays cards, in either ZMW or USD. It uses the secure Barclays Cybersource system.
(We will soon be allowing Maestro Debit cards as well)