I'm with a doubt stupid, but I could not solve

I have two windows, a QMainWindow, and another QDialog, see:

Qt Code: 
  1. NSControl::NSControl(QWidget *parent, Qt::WFlags flags)
  2.         : QMainWindow(parent, flags)
  3. {
  4.         //SETUP OF UI
  5.         ui.setupUi(this);
  6.  
  7.         //SETUP WINDOW(POPUP)
  8.         add = new Add(this);
  9.  
  10.         //CONNECTS
  11.         connect(ui.actionSair, SIGNAL(triggered()), this, SLOT(close()) );
  12.         connect(ui.pushButton, SIGNAL(clicked()), this, SLOT(Apply()) );
  13.         connect(ui.actionSobre, SIGNAL(triggered()), this, SLOT(About()) );
  14.         connect(ui.actionAdicionar_Endere_o, SIGNAL(triggered()), this, SLOT(AddButton()) );
  15.  
  16.         ui.info_tempo->setText("1min");
  17. }
  18.  
  19. NSControl::~NSControl()
  20. {
  21.  
  22. }
  23.  
  24. void NSControl::Apply()
  25. {
  26.         QString tempo = ui.comboBox->currentText();
  27.  
  28.         ui.info_tempo->setText(tempo);
  29. }
  30.  
  31. void NSControl::About()
  32. {
  33.         QMessageBox::about( this, tr("Sobre NSControl"), tr("Desenvolvido por Mateus Vitali") );
  34. }
  35.  
  36. void NSControl::AddButton()
  37. {
  38.         add->show();
  39. }
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Qt Code: 
  1. Add::Add(QWidget *parent, Qt::WFlags flags)
  2. {
  3.         //SETUP OF UI
  4.         ui.setupUi(this);
  5.  
  6.         //CONNECTS
  7.         connect(ui.cancelar, SIGNAL(triggered()), this, SLOT(Cancel()) );
  8.         connect(ui.salvar, SIGNAL(triggered()), this, SLOT(Save()) );
  9. }
  10.  
  11. Add::~Add()
  12. {
  13.  
  14. }
  15.  
  16. void Add::Save()
  17. {
  18.         QMessageBox::about( this, tr("Sobre NSControl"), tr("Desenvolvido por Mateus Vitali") );
  19. }
  20.  
  21. void Add::Cancel()
  22. {
  23.         QMessageBox::about( this, tr("Sobre NSControl"), tr("Desenvolvido por Mateus Vitali") );
  24.         this->hide();
  25. }
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When I click the Add button, it opens the QDialog (add), but when I try to click on the buttons QDialog (Save and Cancel), nothing happens, and one was to appear as I hadplanned QMessageBox

Why?