Godrays appear mostly when there are a lot of clouds in the sky, but where there also some holes in the layer. Clouds have to be very dense, and they have to cast shadow (this has to be ticked in the clouds tab of the atmosphere editor), 40% shadow is good enough. And of course you need to ticvk the godrays box in the sky fog & haze tab of the atmosphere editor.
When you have those clouds with some holes in it, place your sun near the holes, but it must'nt be completely out of the cloud layer. 
You may not see the result in preview mode, you might need to use "final" to see the effect.
And if your clouds are grainy, do as jc suggested.