When it works, you see "Purchase successful" and your balance goes up.
How you pay: in most versions of the app, you pay through the App Store or Google Play, and the summary shows the store fee. Newer versions may let you pay by card, FPX online banking or GrabPay instead. The payment sheet shows the options you can use.
You can also buy Care Credit from the Purchase Care Credits button that shows when you do not have enough for a call or booking.
Care Credit is non-refundable. See Can I get a refund?.