//В Длл:
function CreateMDI: integer; StdCall;
begin
  Result := integer(TForm1111.Create(Application));
end;

//В приложении
var
  FLib       : THandle;
  InitPlugin : TInitProc;
  DonePlugin : TDoneProc;
  CreateMDI  : TCreateProc;
  X: TForm;
.........

procedure LoadMyDll(str:String);
begin
  FLib := LoadLibrary(PChar(str));
  @InitPlugin := GetProcAddress(FLib, PChar('InitPlugin'));
  @DonePlugin := GetProcAddress(FLib, PChar('DonePlugin'));
  @CreateMDI  := GetProcAddress(FLib, PChar('CreateMDI'));
  InitPlugin(integer(Application),integer(Screen),integer (MainForm.Handle),MainForm.OracleSession);
  X := TForm(CreateMDI);
  X.ShowModal;//здесь ошибка
end;