The Azure Flute is an item from Pokemon Diamond, Pearl, and Platinum, and it's used to summon the legendary Pokemon, Shaymin. This item and the Pokemon are not included in Pokemon Explorers of Sky.
If you're looking for a way to summon a legendary Pokemon in Explorers of Sky, you can do so by completing the post-game storyline and then finding the legendary Pokemon in their specific locations.
For example:
* Dialga: You can find Dialga in the Time Gear area of the Temporal Tower.
* Palkia: You can find Palkia in the Space Gear area of the Temporal Tower.
* Giratina: You can find Giratina in the Distortion World.
Let me know if you have any other questions about Pokemon Explorers of Sky!