The account credit balance is like a local bank account allowing them to charge you on a hourly basis, in order to make that work you'll first need to go here, then choose a payment method (Paypal, credit/debit card, paysafecard, or Bitcoin), choose any amount you prefer and finish the purchase.
Then you'll see the money you'll just added in dashboard which means that you're ready to order your server, don't forget to keep this balance above $0 if you want to keep your new server in the future.