It is working, but you need to provide a height and width to the div containing the graph.
<div id="sig" style="width:500px;height:500px;border:1px solid black"></div>
Secondly, the simple example is not very good because the two nodes will be placed on top of each other. So, you might also want to provide x and y properties to each node.
sigInst.addNode('hello',{
label: 'Hello',
color: '#ff0000',
x: Math.random(),
y: Math.random()
}).addNode('world',{
label: 'World !',
color: '#00ff00',
x: Math.random(),
y: Math.random()
}).addEdge('hello_world','hello','world').draw();
Here's a jsfiddle: