>the first 8 lines are ok, the rest, you can explain like a child of 8 years?

Not sure, I'm very weak in English.
It would be better to read about "decorated names" in MSDN.
I can't also understand, is your project C (plot.c) or C++ (plot.cpp)? In first case you shouldn't use 'extert "C"', otherwise this code works without any problems:

// plot.cpp : Defines the entry point for the DLL application.
//

#include "stdafx.h"

BOOL APIENTRY DllMain( HANDLE hModule, 
                       DWORD  ul_reason_for_call, 
                       LPVOID lpReserved
					 )
{
    return TRUE;
}

extern "C" __declspec(dllexport) LPSTR say_hello(LPSTR cText)
{
	return "Hola";
}