I downloaded the default resources as instructed by @Kuro007 and I'm not longer getting the error message when I load Revvolution Omni Hair Shaders. However, I'm having problems with both Christopher hair and Renaissance Curls when I try to apply their own materials to them. I get an error message when I try, the hair remains grey in the Iray previews on both.
Any thoughts on what's going wrong? All help greatly appreciated.