(For iOS 6 +.) In the info.plist add the key "Privacy - Location Usage Description" the Type is String and in Value add the text you want to appear when the user is prompted to enable Location Services for your app.
Hava a look at the Apple Property List Key Reference for more info: https://developer.apple.com/library/ios/documentation/General/Reference/InfoPlistKeyReference/Articles/CocoaKeys.html