Error opening iOS Simulator with iOS 8 beta version: "Unable to boot the iOS Simulator"

After I found some solutions on the web, a combination of procedures worked for me:

1. Close Xcode and iOS Simulator (and all related applications)

2. Install (or reinstall) Xcode6-Beta to Applications folder (direct from the .dmg file)

3. Open Xcode6-Beta and go to Xcode -> Preferences -> Locations -> Command Line Tools and select "Xcode 6.0" (be sure that you've selected the one in Applications folder)

4. Launch the iOS Simulator and go to iOS Simulator -> Reset Content and Settings...

5. Close all the applications and restart your mac (I unchecked "Reopen windows...")

6. Open Xcode6-beta and test the iOS Simulator

Good luck

I also had this problem and found the solution by doing following steps:

1. Initially i downloaded Xcode 6 library in Documentation section under the path Xcode --> Preferences --> Downloads and restart my xcode but didn't solve my problem.

2. Downloaded and installed the iOS 7.1 Simulator under the path Xcode --> Preferences --> Downloads. Restart xcode again and the problem solved.

3. I am facing same issue again next morning. Unsetting the DYLD_INSERT_LIBRARIES variable solves my problem:-

   1. Open terminal and type "sudo vi /etc/launchd.conf" command and press Enter

   2. Press I "Insert" mode and change this

      "setenv DYLD_INSERT_LIBRARIES /usr/lib/libimckit.dylib" 
      

      to this

      "unsetenv DYLD_INSERT_LIBRARIES /usr/lib/libimckit.dylib" 
      

   and press esc to exit from insert mode.

   3. Save changes by typing ":wq" Press Enter
   4. Reboot your system and problem soloved.

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Prior to that I didn't find any iOS version in front of each listed devices but after installation or by unsetting DYLD_INSERT_LIBRARIES variable I found the same in front of each device.

I solved by removing and re-adding the simulators see screenshots here.

https://stackoverflow.com/a/25387715/1565615