If you have an ongoing Monthly Plan and have either run out or are running low on credits you can purchase Top Up credits to get you through to your plan renewal date. You need to keep in mind that top-up credits will expire at end of the monthly billing cycle when your plan renews, so only top up with what you need to get you through, otherwise you will lose them.
How to Top Up;
Go to your Admin tab > Click on Account Summary > Top Up
Then click on the amount you require > Purchase
You will then see a pop up confirmation window telling you how many credits you are purchasing & when they will expire > click Yes