To turn on motion blur, enable RenderMan RIS ROP's Properties | Motion Blur: Allow Motion Blur.

• Xform Time Samples - Output more motion segments for objects to create a smooth blur/motion path in the render. This would help curving motions stay curved as opposed to linear from one point to another with enough samples
• Geo Time Samples -  Output more samples for objects that deform to create a smooth motion path in the render. This would help curving motions stay curved as opposed to linear from one point to another with enough samples
• Shutter Offset - Add an offset to the current shutter times globally

If you need to render motion blur for simulated objects (breaking, falling, bouncing, particles, etc), you need to enable Geometry Velocity Blur on the object node. This provides the correct data for rendering motion blur in RenderMan for these objects as well as deformation blur. This is due to the fact that changing topology takes special consideration for motion blur to work.