在Visual Studio中，Target IOS Device下拉框是禁用状态，无法选择。
Xamarin论坛中有不少关于这个问题的，如下面这个帖子：
http://forums.xamarin.com/discussion/15372/visual-studio-ios-debug-build-failure
Xamarin工程师给出的解决办法：在MAC端，卸载xamarin全部软件，然后重新安装。
这个办法还真有效，跟帖的人都说这样就解决了，me too！
If you are still running into this error, can you try fully uninstalling Xamarin Studio from the Mac Build Host and re-installing? To uninstall the software, please remove Xamarin Studio from /Applications/Xamarin Studio.app and then manually delete the the following -directories /Developer/MonoTouch
/Developer/MonoAndroid
/Library/Frameworks/Mono.framework
/Library/Frameworks/Xamarin.Android.framework
/Applications/Xamarin Studio.app
~/Library/Developer/Xamarin
To re-install, you can run the installer from your Xamarin Store Account
After re-installing, please let me know if you are unable to deploy to the iOS simulator. Thanks!