I am currently running Xcode 6 beta 2, and I can't find the UIKit.framework. I've looked in System/Library/Frameworks as suggested in Apple's reference page
All the other frameworks are there, except UIKit.
Note: I have no issues importing the UIKit framework
UIKit.framework is here
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks