I have installed the QCA designer v2.0.3 tool. I can't find the icon for labeling and substrating, and even the bus layout is also not active.
You can add a label through the Add Objects interface. The official developers have created a page where you can see how to add a label to objects. I recommend that you visit the page linked below to see exactly how to add labels.
Documentation page: http://www.mina.ubc.ca/qcadesigner_adding_objects