The noise function of the clouds is the shader that is plugged-in to the Density Shader input of your Cloud Layer. When in the Atmosphere Layout with your cloud layer selected you can click the Pattern button at the bottom of the Node List to access it. However, as I mentioned, if you are using Easy Cloud you do not have this option, the noise function is internal and not editable. In that case you would need to start over with a "Generic" cloud layer which will expose the noise function so you can edit and manipulate it with things like the Vortex Shader.
- Oshyan