class Ui_MainWindow(QtGui.QMainWindow):
    def setupUi(self):
        self.MainWindow = QtGui.QMainWindow()
        self.resize(800, 400)
        self.setWindowTitle("main")
        self.rowcount = 6
        self.table = QtGui.QTableWidget(self.rowcount, 6, self)
        self.table.setGeometry(QtCore.QRect(20, 20, 700, 225))
        self.table.setSortingEnabled(True)
        self.addRowButton = QtGui.QPushButton('Добавить строку', self)
        self.addRowButton.resize(self.addRowButton.sizeHint())
        self.addRowButton.move(30, 250)
        self.sethaeders(self.table, headerlist)

        self.show()
        QtCore.QObject.connect(self.table, QtCore.SIGNAL("clicked(QModelIndex)"), self.addRowButtonClicked)#неверный первый (возможно и второй) аргумент
        QtCore.QObject.connect(self.addRowButton, QtCore.SIGNAL('clicked()'), self.addRowButtonClicked)

    def sethaeders(self, table, haederlist):
        index = 0
        for item in haederlist:
            self.table.setHorizontalHeaderItem(index, QtGui.QTableWidgetItem(item))
            index += 1

    def addRowButtonClicked(self):
        self.table.insertRow(self.rowcount)
        self.rowcount += 1

    def __init__(self):
        super(Ui_MainWindow, self).__init__()
        self.setupUi()

def main(args):
    app = QtGui.QApplication(args)
    window = Ui_MainWindow()   
    sys.exit(app.exec_())

if __name__== "__main__":
    main(sys.argv)

self.table.currentItemChanged.connect(self.cell)

def cell(self):
        if self.table.currentRow()+1==self.table.rowCount():
            self.table.insertRow(self.table.rowCount())