Hi, had the same issue. Couldn't install plugin for Rhino 6 version OctaneRenderForRhino6_3_08_1_92.
After deleting Rhino 5 from my system and trying to drag the plugin to Rhino 6 I saw it was trying to call Rhino 5, in Rhino 5 install folder.
*.*rhi seemed to be liked to wrong version of Rhino ...