See the following link: http://ftbl.ma/fmcdev
You must be running at least iOS 8.0.
You can buy the game from the iTunes App Store via this link: https://itunes.apple.com/app/football-manager-classic-2015/id944235778
Please create a new thread on our bugs forum http://community.sigames.com/forumdisplay.php/475-iOS-Bugs-Forum and we'll do our best to talk you through resolving any problems you might be having with installing the game.
Such issues are rare, however they're also device specific so we may need to ask you a few questions in order to resolve them.