Dear Ana and Tasos,
Just to clarify, you can install a 32-bit version of MATLAB on a 64-bit machine and still use the *.mexw32 file. If you’ve installed a 64-bit version of MATLAB, then you’ll have to recompile the S-Function in 64 bit to create a *.mexw64 file. While I know several people who have successfully done this, I don’t have experience myself. I suggest you start with the files in the “Simulink\Source” folder of the FAST archive.
I hope that helps.
Best regards,