BoxJump is designed for iOS 8, which Apple released on September 17, 2014. Due to technical differences with iOS 7 (and earlier versions of iOS), BoxJump is not compatible with iOS 7, and therefore will not show up in the App Store if you have not yet updated your phone to the latest version of iOS.
To check your current version, open “Settings” on your iOS device, then tap “General,” then tap “About.” The installed version of iOS will be listed in the row labeled “Version” – it should read “8.0.0” or higher.
If you need to update your software, you can tap “General” in the top left corner, then “Software Update” (or if starting from your home screen, tap “Settings,” then “General,” then “Software Update”). You should then have the option to install the latest version of iOS, at which point you will be able to see and download BoxJump on the App Store.