> Could you explain me the correct way for calling them and is the definition of the external functions done with copying the given snippet into "Libraries/External function declarations" in the same form how it is at the moment? >
It sounds as if you need to spend some time studying C. Learn what the extern definition does, and how to call external libraries from C-code.
In your case the S-Function acts as a wrapper for converting the C syntax that Simulink expects/uses to that expected/used by your library. That conversion is done in C and you need to have a good understanding of how to do it before writing the wrapper S-Function.