Can't select iOS simulator in Xcode

Press "Window" and then "Devices", check if they appear there and if not - click the "+" icon in the lower left

Go to File -> Project Settings -> Info. Change the deployment target to a lower or higher version.

Most probably you do not have a simulator installed for the current deployment target. You can do so by going to Xcode Preferences -> Downloads. In the Components section, you will see the options for downloading the simulators you already don't have.

Faced the same issue many times and found this solution. Hope this helps. :)

I suspect that you are having issues communicating with CoreSimualtorService due to renaming Xcode.app or similar. This is mentioned in the Xcode 6.0 and 6.1 release notes. Unfortunately, the only workaround at the moment is for you to rename it back or reboot.

Update:

Note that for current (as of this update) versions of Xcode (7.2+), renaming should work ok. For best results, only use one version of Xcode at a time. If you start another version of Xcode while one is running, the previous one will loose access to simulator devices. Restarting it will cause it to have access to the simulator devices (and of course the other one to stop). Basically, the last one wins.