I'm guessing you already find it out, but just for the record: the UIKit.framework is available only on iOS platform. The path /System/Library/Frameworks/ contains the frameworks that your Mac OS X has installed (not your Xcode).

The iOS frameworks that are used in iOS projects, at least when using Xcode, is the following:

/Applications/Xcode6-Beta4.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS8.0.sdk/System/Library/Frameworks

The Xcode6-Beta4.app part of the path is the app name of your Xcode in your Applications folder, that can be somewhat different if you have another version installed. Also, theiPhoneOS8.0.sdk can be different if you're using other SDK version (iOS 7.0, for example).

EDIT: If the UIKit.framework is there, then you probably changed some configuration in your Xcode. The easier way to fix it is probably by deleting it and reinstalling.