Smelly Jelly Achievement
Get to chapter 4
In other words, clear the previous chapter. Each chapter consists of 9 normal levels, and once you have beaten at least six levels in a chapter, you will be able to select the right arrow at the right side of the page and advance to the next chapter. The achievements will pop once you enter that new chapter. It is not required to complete the three bonus levels at the end of each chapter either, but you will want to complete them if going for "The Hunter" achievement.