I would suggest you to use QPushButton instead of QToolButton. Here is a working example:
QMainWindow window;
QVBoxLayout *layout = new QVBoxLayout();
QPushButton *button = new QPushButton( "Menu button" );
QMenu *menu = new QMenu();
button->setMenu( menu );
menu->addAction( "teste1" );
layout->addWidget( button );
window.setCentralWidget( new QWidget() );
window.centralWidget()->setLayout( layout );
window.show();