Add text to QPlainTextEdit in PyQT (the result is a status log)

I am trying to build a StatusLog, every move that happens on an application. For example, the user clicked in the button1, the progressBox must receive a string like "user clicked the button1".

I've searched and found a link but I can't put that working out, I don't know how.

Link: Add more than one line to a QTextEdit PyQt

Everytime I try it gives me AttributeError: 'QPlainTextEdit' object has no attribute 'insertText'

from PyQt4 import QtCore, QtGui
import sys, os
from math import ceil
from PyQt4 import QtCore, QtGui, QtNetwork
from qrtools import QR

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    _fromUtf8 = lambda s: s


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):        
(..)
        
        #The button
        self.groupBoxAuthbtn.setSizePolicy(sizePolicy)
        self.groupBoxAuthbtn.setTitle(_fromUtf8(""))
        self.groupBoxAuthbtn.setObjectName(_fromUtf8("groupBoxAuthbtn"))
        self.horizontalLayout_2 = QtGui.QHBoxLayout(self.groupBoxAuthbtn)
        self.horizontalLayout_2.setObjectName(_fromUtf8("horizontalLayout_2"))
        self.btnAuthenticate = QtGui.QPushButton(self.groupBoxAuthbtn)
        self.btnAuthenticate.setEnabled(True)
        self.btnAuthenticate.setText(QtGui.QApplication.translate("MainWindow", "Auth from Webcam", None, QtGui.QApplication.UnicodeUTF8))
        self.btnAuthenticate.setObjectName(_fromUtf8("btnAuthenticate"))
        self.horizontalLayout_2.addWidget(self.btnAuthenticate)
        self.horizontalLayout.addWidget(self.groupBoxAuthbtn)
        #self.btnAuthenticate.clicked.connect(self.handleTest)

(...)

        # THE STATUS LOG
        self.groupBoxProgress = QtGui.QGroupBox(self.centralwidget)
        sizePolicy = QtGui.QSizePolicy(QtGui.QSizePolicy.Expandi