mex error mac osx 10.7 error

I want to use one of the later versions of Xcode as my C compiler in MATLAB 8.1 (R2013a) or MATLAB 8.2 (R2013b); however, I get the error

 xcodebuild: error: SDK "macosx10.7" cannot be located

when I read this http://www.mathworks.com/matlabcentral/answers/103904-can-i-use-xcode-5-as-my-c-or-c-compiler-in-matlab-8-1-r2013a-or-matlab-8-2-r2013b and follow the direction , I my error became xcodebuild: error: SDK "macosx10.8" cannot be located

when i change to 10.9 the error became /Applications/MATLAB_R2013a.app/simulink/include/simstruc.h:148:10: fatal error: 'string.h' file not found

can somebody help me and tell me what happened?