Bonus Bets must be used in whole as they cannot be split into multiple wagers. When you win a bet for which you have used a Bonus Bet, only the prize amount of that bet will be returned to your wallet. Any Bonus Bet values used are not included in settlement returns.
Click on the information icon next to your bonus funds. If you have fulfilled the wagering requirements for the bonus, your bonus funds should reflect in your available balance. When the progress bar hits 100%, you can convert your bonus funds to cash by clicking "Redeem Bonus."