The Rhino/Grasshopper intergation of Orkestra is still in it's beta phase. This is why it doesn't get installed automatically. If you wish to install Orkestra for Rhino, all you have to do is drag and drop the OrkestrRGH.rhp file (located in %programfiles%/orkestra/orkestra) onto as Rhino window! (Compatible with Rhino 6 and 7).
Drag and drop the OrkestraRGH.rhp file to install Orkestra for Rhino !