I’ve updated the Cycles for Rhino plug-in to v0.0.2. This should load in most recent Rhino WIP without problems. If you find otherwise, please let me know.
If you don’t know what this is for then it is safe to skip this plug-in for now
Remember that this plug-in isn’t officially released yet, nor is it currently part of Rhino WIP (but hopefully will be sometime this year). The main focus is on the Raytraced viewport mode and everything related to it. That said, this plug-in should work.
The box is whiter because there’s difference in how skylighting is done. IMHO Cycles does it correct - white light on white surface = white.
The gray ground plane is because there’s a bug with just skylight and shadows-only ground plane (currently working on it here RH-33841). Add a primitive light to get rid of the gray shadow on the shadows-only ground plane.