I just upgraded to Xcode 6.3 and they offered something new to the Playgrounds. If you make a new playgrounds and you open the project navigator, you will see a Sources folder and inside that there is a "SupportCode.swift" file. At the top of that file it reads
This file (and all other Swift source files in the Sources directory
of this playground) will be precompiled into a framework which is
automatically made available to <PLAYGROUND_NAME>.playground.
You have to add public access attribute to your classes, methods and properties in source folder to make them accessible from main playground file as they treated as separate module by compiler