You need to collect six secret puzzle pieces throughout the game. Then after you complete the main story, there will be a new option "Secrets" in the main menu. Once you have all six pieces you can visit the airship and the cheevo pops immediately.
There is an achievement tied to each of the pieces so instead of me writing new guides for them all, you can find them here:
Chapter 2/4 (Clap at the plant multiple times)
Chapter 5 (Poke a cactus multiple times)
Chapter 6/10 (Get the four birds next to each other)
Chapter 7 (Spin the volcano rapidly)
Chapter 8 (Make a mini fishing lake and catch a fish)
Chapter 9 (Rebuild the snowman)