1. You will need to register to a serice that allows you to install signed app's.
Examples: AppIPA - Signulous - Buildstore - TutuApp (Links in link section)
Note: Free services are good, however are normally filled with adverts.
2. Once registered. You will need to install their profile on to your iPhone / iPad using their instructions.
3. Find Kodi in their "App's" section.
4. Download and install.
5. Then follow the "INSTALLING THE WIZARD / PORTAL ON KODI - ALL DEVICES" guide as normal.