I see that Xcode6 has removed the "Devices" tab from the organizer and moved it to a separate "Devices" tab in Window > Devices.
I do not see an option for "Add device to Member Center" from the Devices screen or the Devices > Provisioning Profiles screen.
How can I add a device to Member Center and thus get the correct provisioning profiles installed in Xcode 6?
In your build settings, set your provisioning profile to automatic. Code signing identity to automatic as well.
In some cases the xCode tries to fix both the main target and the "tests" target, but fails to properly sign the tests target. In such case, for demos downloaded from the internet, I just delete the tests target.