You don’t install a Grasshopper plugin in Rhino. You install them in Grasshopper.
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It would be nothing to write home about if draggin into Rhino window itself started the installation-opening sequence of a grasshopper object or document
Same with scripts
Also Rhino V6 SR2 is way out of date. The current Service Release is 29, making yours 27 months out of date.
Please update to the current Service Release.