Dear Mehdi,
The S-Function has changed from “FAST_SFunc.dll” to “FAST_SFunc.mexw32” in MATLAB R2006a and newer.
Included with the FAST files of the NREL 5-MW OC3-Hywind model is a *.mexw32 version of the FAST-SFunction for 32-bit Windows versions of MATLAB. As has been described in many posts on this forum, you can install a 32-bit version of MATLAB on a 64-bit Windows 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. Many people of tried this and ran into trouble running the *mexw64 file after compiling. One post recently discussed their success. While I haven’t tried it myself, you can see their comments here: Compiling FAST/FAST S function on Linux x86_64.
Best regards,