When I try to do anything in Playground, this pops up before I even finish typing a word.
Error running playground. Failed to launch iOS stub for playground: The ope
Currently the Xcode 6 beta needs to reside in \Applications for the playground and simulator to work. After you move it there you need to reboot.
Your firewall is blocking the debugger, you need to allow it to connect:
TCP out 127.0.0.1:63748
Or allow the debugserver
process open access (whichever you, or your firewall, prefer(s)).
Upon enabling it, and restarting Xcode everything should work like a charm.
Note/Update:
If you're having trouble finding, or allowing, the debugserver
simply try, temporarily, disabling your firewall and restart xcode. Does the playground work? Oh, and don't forget to make sure you've got the assistant view open and the output box showing.
This error popped up for me when I closed the simulator before Xcode finished attaching my application to it. Opening the simulator and running the app again solved this problem.