Hello,
Can you please give more details?
Are you able to top-up and then unable to buy subscription time, or are you unable to top-up?
Do you have an error message?
If you can't top-up, please try another payment method, and if nothing works (or if you don't have an alternate payment method), please submit a ticket to the support team
http://support.my.com/games/kbPage/6/