Here is a workaround for setting up mex in MATLAB2015a within MacOS Yosemite.

You need to install two software packages: Command Line Tools (CLT) and gfortran.

1. Installation of CLT

Simply download the following and install. (You need Xcode 6.3 or higher as a prerequisite).

https://developer.apple.com/downloads/index.action?name=for%20Xcode

2. Installation of gfortlan

go to the following site and download gfortran 5.0

http://hpc.sourceforge.net/

Direct download link for gfortran 5.0

You will have gfortran-5.0-bin.tar in your downloaded folder.

Open terminal console and type the following for gfortran installation

sudo tar -xvf gfortran-5.0-bin.tar -C /.

Note that don’t forget “/.” at the end of the command line.

You will see some log messages copying files into your /usr/local/bin and related folders.

Done!

For verification, just type “mex -setup FORTRAN” in MATLAB console. This should show you the following message.

MEX configured to use 'gfortran' for FORTRAN language compilation.
Warning: The MATLAB C and Fortran API has changed to support MATLAB
	 variables with more than 2^32-1 elements. In the near future
	 you will be required to update your code to utilize the
	 new API. You can find more information about this at:

http://www.mathworks.com/help/matlab/matlab_external/upgrading-mex-files-to-use-64-bit-api.html.