Click on "Mac HD" and click on "Library." In "Library," find the "Application Support" folder and click on it.
Navigate to the "Digidesign" folder. In the "Digidesign" folder, click on the "Plug-ins" folder. Usually when you install a new plug-in, Pro Tools automatically installs it in this folder.
Find the plug-in you wish to install. Drag the plug-in icon and drop it into the opened "Plug-in" folder. This will start the installation process. When the plug-in is installed, click "OK."
Go to the "Start" menu and click on "Computer." Double click on the "C:" drive.
Double click on "Program Files." In the "Common Files" folder, find the "Digidesign" folder. Inside the "Digidesign" folder, click on the "Plug-ins" folder.
Find the plug-in you wish to install and drag it and drop it into the opened "Plug-ins" folder. This will start the installation process. Click "OK" when the installation process is finished.