Using the Denoiser in Maya requires a number of steps.

Step 1.

Ensure that in the RenderMan Preferences, Launch denoiser is ticked (it is on by default)




Step 2

In RenderMan's AOV tab in ARender Settings, Select Beauty and untick As RGBA and enable Denoise.  You can then amend the denoiser parameters.




Step 3

Launch a Batch Render from the RenderMan menu and once the frame(s) have been rendered, the denoise process will begin.