To add a node left click on whitespace.
To add an edge drag from one node to another.
To move a node hold ⌘ (Command) while dragging it.
Once moved, vertices stay fixed in position (shown with dashed border).
To unfix a node, double-click it.
To delete a node or edge right click on it.
To change the color of an edge or node, click on it.
To adjust edge probability, click and drag up/down on the edge.