Open the app and search for the I-Chu game icon, or search for アイ★チュウ. Searching for "I-Chu" works too.
Download it and you're done!
You need to have a Japanese Apple account to be able to download the game, using a non-japanese account will force yout to put in a credit card. If you are unsure how to make one, refer to this tutorial.
If you are done, or already have a Japanese Apple account, search for アイ★チュウ and download the game.