This is the full procedure for installing new plugins in Grasshopper.

Open “Grasshopper Components Folder”

You can easily find this folder by using Grasshopper File menu and clicking on Special Folders -> Components Folder.
This folder contains all the additional plugins you want to use inside Grasshopper. Your new plugins most likely go here!
The files have a green icon and their extension is .gha.

There is also another “special folder”, the User Object Folder. This folder contains user-created components, you can recognize them from their yellow file icon and their extension is .ghuser.
Some plugins, especially in their early development, are released as collections of user objects.
If you download a Grasshopper plugin and see such yellow icons then you will have to put the plugin files inside the User Object Folder and not inside the Components Folder.

Copy plugin files inside the Components Folder

If you downloaded just a .gha file simply copy it in the Components Folder.
If you downloaded a .zip file it might contain several files (eventually also some .dll files): unless specified by an installation guide, extract all the files inside the Components Folder.
Pay attention: if your download consists of .ghuser file(s) then you have to place such file(s) in the User Object Folder!

In this tutorial we are installing a new Grasshopper plugin called “3D Graphic Statics”.
Please notice that plugin files have a publication date (underlined in red in the following image): you should always select the most recent version of the plugin.

This plugin, for example, comes with installation instructions.

Inside the .zip file there are three folders: the important one is the “installation” folder, which contains the plugin files. In this case there is just one .gha file, GraphicStatics_V01.gha: this is the file that goes inside the Components Folder.

Unblock plugin files!

For security reasons, when you download files from the internet your OS will save them as blocked content.
Grasshopper cannot load blocked files so you must manually unlock them!

Once you unlock the file, the red area in the previous image looks empty: this indicates that the file is already unlocked and you don’t have to worry because Grasshopper will be able to read it.

Restart Rhino and Grasshopper

Grasshopper plugins are loaded when Grasshopper is executed, so right now the new plugin won’t show up in the toolbars area.
You must restart Rhino and Grasshopper.
Finally you can see the new plugin installed and running.