The intention of the insert mode is to make it easy and intuitive to put in vertices and edges in the graph.
The only action that is needed to put in a new vertex is to click with the mouse on the position where you want the vertex. The position where you click needs to be empty if it is not the object at the place you click will be selected and no new vertex will be created. To create an edge between two vertices, press the mouse button in the middle of one of the vertices and drag the mouse with the mouse button down, until the mouse pointer is in the middle of the other vertex and then release the mouse button.
To get a menu with commands click with the second mouse button in the graph window. The commands are different depending on the selection of vertices and edges.