i keep getting an error under 'Unable to locate a C-compiler required by Stateflow and MATLAB Function blocks'

Hi, I keep receiving an error message in SIMULINK when trying to use MATLAB function block. I installed microsoft VS 2013 and 2012. with 2012 I succeeded to configure mex -setup and MATLAB showed 'successfully recognized' message; still I can use MATLAB function block in SIMULINK because I receive that cursed error message!! please help it's pretty urgent my project is nearly due.