You can add vertices several ways. The easiest for me (the one I use most often) is, after you have selected your object, go to Sub-object mode and select Edges. Then one of the Modify buttons farther down should become Cut. Select it then click on the edge where you want the vertice to be and then click on an opposing edge. You should see a dashed line appear when you move the cursor across the polygon, this will show you where the new edge will form.
Hope this helps.
Jer
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