I have created a light as following:
var light:DirectionalLight=new DirectionalLight();
scene.addChild(light);
and created a mesh as following:
var sphereGeometry:SphereGeometry = new SphereGeometry(100);
var sphereMaterial:ColorMaterial = new ColorMaterial( 0xff0000 );
var mesh:Mesh = new Mesh(sphereGeometry, sphereMaterial);
scene.addChild(mesh);
But the sphere is just a plain red sphere. It is not illuminated by the light.