Some graphs have isolated vertices, or isolates, which are vertices that are not connected to any edges. If your graph has isolates, you have to take special action to get them to appear in the Graph Pane.
Step 3 is required because by default, a vertex on the Vertices worksheet is shown in the Graph Pane only if it is connected to at least one edge.
(There is a reason why NodeXL works this way. If every vertex listed in the Vertices worksheet appeared in the Graph Pane, even those not connected to edges, then you would not be able to delete an edge from the Edges worksheet without checking whether the edge's vertices were now isolated, and then removing the isolates from the Vertices worksheet if necessary. That would be quite burdensome.)