You need to install a compiler appropriate for your R2014a MATLAB release and your operating system. The list of supported compilers for your release is athttp://www.mathworks.com/support/compilers/R2014a/win64 . If you are using Windows 7 then I suggest you install SDK 7.1. If you are using Windows 8 then you SDK 7.1 will still work but you will need to go through a bit of trouble to get the right .NET installed. If you are using Windows 10 then your are unlikely to be able to get a compiler to work with R2014a.
Because you are trying to write under the MATLAB installation directory, you would need to run as administrator to be able to use mex to create files there.
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