Here is a link to the Pixar website that has the fog.slo file and general instructions on how to use it. For our project, just grab the fog.slo file and place it in the top level of the shot directory you're working in.
1.) Whatever objects you want to fade into the background, you have to import the material into the Maya scene and attach it from the Hypershade.
2.) Select the shading engine of the material and add the "Atmospheric" attribute from the extra renderman attributes pulldown.
3.) Attach the fog.slo to the Atmospheric attribute.
4.) From there, you can select the Atmospheric node in the hypershade and adjust the color the object fades into and the distance from the camera that you want the fading to start.