From the menu, go to Options-Settings, then Unusual Games tab.

If the game is one of those on the right, click on it and turn on Unusual game active. You can click Custom/Bonuses/Side Bets to change betting.
If it is not in the list, click Custom and then Custom Bonuses/Side Bets. You can create your own games on the Custom screen. You can base this on one of the included templates, which also show how various side bets are already defined.