For this achievement, go into the level select, and scroll over to "Coming Soon." Click on the "i" in the top right, and you should get a message about a hidden treasure. Keep scrolling right by click on the right arrow until you pass "Coming Soon" 3 times. You should see a coin. Click on that and you will get both this achievement and a coin that counts for the secret coins achievements.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.