The shader tweaking is not visual. You write shaders in a standard text-editor. Or you can use the VirtuaLightGUI that comes with the renderer, but you still have to type the code for the shader.
BUT, if you wait for a while (think a month at least here) I’m going to release a great (imo) material editor for VS (that’s the shader language used in VL). This far, I’m done with the backend, but I’ve only just started to sketch out the look and feel of the GUI for it. Anyway, when you get your hands on this, you’ll be able to create very complex bitmap-based shaders with some procedural components as well without having to write one single line of code.