The site's only currency is USD. Task prices can go below a cent, so amounts are shown with up to six decimals when needed (e.g. $0.0026); round amounts use two decimals.
IMPORTANT: no payment provider is connected yet. The top-up methods on the balance screen go live once a provider account is set up; until then real money cannot be added.
The balance screen has a test top-up button; it is only for trying the system out and is not real money.
If you have a coupon, use it under Balance > Coupon. Each coupon can be used once.