Authorization is required to install the packages.
I am running OS X 10.11 Beta (15A216g) and Xcode Version 7.0 beta 3 (7A152u).
From the Xcode > Preferences > Downloads panel I am able to download all of the documentation just fine. When attempting to download the iOS 8.1 Simulator, the iOS 8.2 Simulator, and the iOS 8.3 Simulator I receive the "Could not download and install iOS 8.3 Simulator. Authorization is required to install the packages." (The error message displays the correct iOS version per simulator).
I have removed and re-added my developer ID several times, each time the login process being approved.
Has any one else come across this issue and mind sharing your findings/solution?