The run destination My Mac 64-bit is not valid for Running the scheme '*****'.

问题：The run destination My Mac 64-bit is not valid for Running the scheme 'Hello Objective-C'.

解决办法：How to change the SDK: 
1) Click on the project icon in the left hand panel. 
2) In the right hand panel that appears, select Build Settings » all. 
3) You'll see the option to change the SDK. Screenshot
4) 把Base SDK項改成ios 5.0（这里要看你电脑的版本了）

如图：