...

You can attach materials (bxdf's) to selected objects using the shortcuts in the Object Context Menu:

This will attach the selected material to the selected objects in the outliner.

You can also attach a material to an object in the usual way under the material properties within the Material Properties tab .

Click New:

Image RemovedImage Added

Then click Add RenderMan Nodetree 

Image Added

Nodetree and select the RenderMan material you want to use.

Image RemovedImage Added

RenderMan shading networks will always terminate at a RenderMan Material Output node. Here's an example shading network:

...