There are a couple options for calling a custom function from your model.
The first is to use a MATLAB function block. In a MATLAB Function block you can use a function called coder.ceval in order to call your custom code. The examples at the bottom of that page are for using the MATLAB Coder product, but the MATLAB code is specifies is exactly what you would put in the MATLAB Function block. As long as the files containg the function declaration and definition are in the custom code section of the configuration parameters, you should be all set.
The more popular way to incorporate custom code is to use something called S-functions. Lots of information on how to use S-functions can be found starting from this page:
My suggestion would be to look at the Legacy Code Tool, as I have found that to be the easiest to use and extend in the future. If you go that route, you do not even need to include the custom files in the custom code section. Whenever someone uses the block for your custom function, the source and header files will be included automatically in the code generation process (assuming you followed all the steps).
Please take a look at the above documentation pages and let me know in the comments if you have any questions.
