Go to My Wallet and Click on Add Balance:
Select your new test credit card and click Next:
Enter the payment amount in the box below and click Pay:
Now your account is loaded with test credit. Remember this is test credit for test purposes, you can not make LIVE topups with it. Only test topups.
If you do not load your wallet in test mode you will get this error in the API:
"message": "Not enough available balance to make this transaction",