I had the same problem with Xcode6 GM and found the error in the system.log. When communicating with the developer portal, my name contained special characters like è, ä, ü, ß etc. After the change to ae, ue and ss I could use my AppleID and all certificates were for the use of Xcode6 downloaded.
Check your "system.log" file with your konsole.app and check if you have an DVTDeveloperAccountErrorPortalResponseText in your communication as the following:
DVTDeveloperAccountErrorPortalResponseText=<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>firstName</key>
<string>René</string>
<key>lastName</key>
<string>Bärnshofer</string>
If yes, change your developer account name in the developer portal. That solved my problem.
But I guess there is an issue in the communication of the developer portal. In Xcode5.x all my accounts are working fine and my userLocale is correct. In Xcode6 my locale is US-EN and so I guess the special characters does not match or could not processed. I opened a bug report for that. Maybe Apple will fix the issue