In Xcode 6.3 and later (including Xcode 7 and 8), console output appears in the Debug area at the bottom of the playground window (similar to where it appears in a project). To show it:
Menu: View > Debug Area > Show Debug Area (⌘⇧Y)
Click the middle button of the workspace-layout widget in the toolbar
- Click the triangle next to the timeline at the bottom of the window
Anything that writes to the console, including Swift's
println in Swift 2 beta) shows up there.
In earlier Xcode 6 versions (which by now you probably should be upgrading from anyway), show the Assistant editor (e.g. by clicking the little circle next to a bit in the output area). Console output appears there.