To use your bonus bet, put your desired selection in the bet slip. You will see in the bet slip an option to use a bonus bet for this bet.
A bonus bet is basically free swing from a bookie. You place the bet and if it loses you don't lose any money. Bear in mind the Bookie will keep the bonus stake. eg you put a $50 bonus bet on a horse paying $2. The horse wins and the return is $100, the bookie keeps the $50 bonus stake and you keep the other $50.