To turn on Denoise go to…
render settings> renderman> denoise> choose frame or cross frame
Frame will automatically apply the settings needed. Since it is applied as a processing tool, you are required to batch render. It does not support interactive rendering.
Choosing Cross Frame
Cross Frame is better suited for animation sequences. Filtering takes into account the pixels from frames before and after the frame currently being rendered.
In the image below you can clearly see the difference denoising can make. Left being before, and right before after it has been applied.