Welcome to the Roundup 58 - Retro with four games! After you find the ten monkeys in the chapter, look in the inventory. You will find a code there and note it down - after four chapters there is a bonus one where you need all four codes. To get to the bonus, click on the "next stage" arrow after completing the last stage in the Roundup.
Note: If the games don't load in your browser try Chrome.