Just drop the plug-ins on top of the closed System Folder. You will get a dialog message similar to the ones below.
Click OK and the Finder will create a new Contextual Menu Items Folder and drop the appropriate items into it. Some plug-in components are library files and will be placed into the Extensions folder. The dialog box will vary depending on the plug-in components.
For any subsequent plug-ins, follow the same procedure and the Finder will place the items where they belong.