I updated Xcode to the 6.1 version and now I don't understand why I can't select the simulator in the top left corner like in the previous version when I want to run my application. The only choice that appear is a general "iOS device".
I finally found the solution. When I added a simulator in the devices menu (Shift+Cmd+2) I got this error: "Unable to determine SimDeviceSet, set_path=/Users/MyUser/Library/Developer/CoreSimulator/Devices" I checked the directory and there was not the folder "CoreSimulator", so I created it. I restarted XCode and now there are all the iOS Simulators selectable for running my app.
Thank you all.