Once you are in the menu to choose chapters, go to the end with the two last chapters (It would be night time), and behind the available options, upper-left corner, you will see an illuminated window with a butterfly/moth sticker. You must click on the moth to enter the secret chapter.
Did you download the new update? The one posted in the log where we are talking right now? Jaxter's chapter is the last one from the previous update. You can't access the next and final chapters, included the bonus chapter, if you didn't download this one.
