With earlier versions of Unreal, building a project for iOS wasn't particularly easy. However, with UE4, you can now build and run your project with just one click. You have two different options for how you can build your project into your device: launching and packaging.
Launching allows you to launch only the current level to test it in your device. It can be accessed from the top shelf of your editor, and when you press the little arrow button, it will give you a list of the available devices that you can launch the game into.
Packaging allows you to build an *.ipa file, you can use any other application to push this file into your device to test it.
In addition to packaging the game, you need to navigate to File | Package Project | IOS and make sure that, while the packaging sub menu may look different for you, it will always contain the platforms supported by your OS. Currently, Mac supports only iOS and Mac, and Windows supports Windows, iOS, and...