Why the need for stdcall? Have you tried without it?
Are you compiling in Delphi?

Try a simple example first, such as:

var
CBackProc: Procedure(src: Longword);

procedure test(src: Longword);
begin
WriteLn('Hello')
end;

begin
CBackProc:= Test;
CBackProc(src); // call it
end.