Hello,
Before lunching the Simulation or the compilation, you have to compile the C code firstly. For examle, I have a C file nammed test.c that contain the description of my algorithm. And I call the function in the S-Function. I put the "test" name in the S-function name in Simulink. Before we run or execute the simulink, I compile the C code with this syntax in the matlab command mex test.c to create the Mex File. And then, we run the Simulink. I don't have any error about the existing of the function. Be sure your the path of your s-function is setted in the Matlab Path.
