cough Documentation Typo cough
The actual method name is "Material"

Getting nodes is only possible in P6 using

lightMat = theLight.Material("Preview")<br></br>tree = lightMat.ShaderTree()

If you want this to also work in P5 (or avoid having to navigate the shader tree in P6), you need to set the node input to animating, which will create a parameter on the figure / prop, which can then be manipulated by a callback on the light parameter.

Take a look at the "parmCallback.py" sample script to see how to set up callbacks.