Hi, David.
I am not very familiar with Simulink’s external mode.
FAST_SFunc is normally compiled as a mex function (dynamic library) with the create_FAST_SFunc.m script found in the FAST archive. OpenLoop.m will call the already-compiled mex function. The FAST v8 ReadMefile has a section explaining the FAST-Simulink interface.
From what I have read about Simulink’s external mode, you need to create a target application from your Simulink model. This target application would need to include the mex function and the FAST dlls (FAST_Library and MAP). I would guess you’d have to recompile them in a special way, but I don’t know what that is.
I would suggest you get your model working in the normal Simulink environment first (without external mode, connecting to real-time applications, or target hardware).