To buy Pay as you go credits you need to be logged into the app. On the left menu click the "upgrade now" as highlighted below.
​
​

​
You will be presented with the below screen after clicking on the pay-as you go option
​
​

​
Select one of the default options and Checkout to buy the Credits or Enter the amount and Checkout to buy the Credits
​
​
​
Checkout
​
​