NSTabView and NSSplitView are busted in beta 4 of Xcode 6 when using OS X Storyboards. If you are using Swift you can drop this file into your project to get them working until there is a fix.

StoryboardExtensionsBeta4Fix.swift

//
//  StoryboardExtensionsBeta4Fix.swift
//
//  A workaround so that you can use SplitView
//  and TabView items in OS X Storyboards on
//  10.10. Just drop this in your Swift project.
//  
//  Hopefully the need for this goes away in soon
//  as it used to work.

import Cocoa

extension NSSplitViewItem
    
    {
    
    var controller: NSViewController! {
    
    get {
        return self.viewController
    }
    
    set {
        self.viewController = newValue
    }
    }
}

extension NSTabViewItem
    
    {
    
    var controller: NSViewController! {
    
    get {
        return self.viewController
    }
    
    set {
        self.viewController = newValue
    }
    }
}