Volumetric light is an independent layer in Smode, which mean that depending on where you put it in the tree it will be displayed differently.
To make it work, first make sure that your
Illuminate 3D renderers
are configured to display volumetric (go in advanced parameters of the lights)
Then you can instantiate your volumetric light (with right click or ctrl+space and type "volumetric").
In order to see your volumetric blending well with the geometries, put it after the geometries and enable the depth test in it's parameters.
You can check the tutorial part of the documentation to see an example using the Volumetric.
Else check this quick tutorial to see how to do a simple set up :