I\'ve been trying to install iOS simulators 8.3 & 8.4 on Xcode 6.3.2 an 6.4 beta respectively. In the first case is not even an option to download this simulator while in Xc
The iOS 8.3 Simulator runtime is includes in Xcode 6.3 (which is why it isn't offered as an additional download).
The iOS 8.3 Simulator runtime should be downloadable from Xcode 6.4, as you show. Your dialog box reports that there was an authentication problem when attempting to install the package. You need to provide administrator credentials when installing the package.
The reason you are getting your "iPhone 6 runs iOS 8.2 which is lower than -project name- minimum deployment target." errors is because you are selecting an iPhone 6 with iOS 8.2 on it and not an iPhone 6 with iOS 8.3 on it.
Use 'xcrun simctl create' or Xcode's Devices window to create the iOS 8.3 devices you need.