I\'ve been trying to build on a Cocoa app that uses Swift and Storyboard in Xcode 6, but how can I use NSToolbar there?
In Xcode 5 and xib, you can add
I had this very same problem. The solution works for both Objective-C and Swift projects.
Working with Storyboards on OSX, it creates an instance of NSWindow and segues to another NSViewController as its Window Content Segue, as you described.
On your ViewController, create a standard IBAction to receive the action when the toolbar is clicked. To wire it up with the NSToolbar, just control-drag (or leftClick-drag) from your NSToolbarItem to the FirstResponder object, as shown on the picture below.

This will open a HUGE list of available connections. Your IBAction will be on that list. Just selected it and you're good to go.
Hope this helps!