If you have Cycles for Rhino (CfR) set as current renderer switch to Rhino Render. CfR will not be in the final V6 product, so best is to test and use Rhino without it. If you need a different resolution from the active Raytraced viewport you can use ViewCaptureToFile or ViewCaptureToClipboard. Both do modal rendering if necessary, and are much faster than rendering to the RenderWindow with CfR.