To install a plugin that can be accessed from within WebDesign, simply create a folder called 'Plugins' in the same folder as the WebDesign application and place your plugin(s) in the that folder. The next time you launch WebDesign the plug-ins contained in the plug-ins folder will show up at the bottom of the Tools Window (Window->Tools Window).
You can organize plug-ins using sub folders within the 'Plugins' folder. These sub folders will appear in the popupmenu near the bottom of the Tools Window. You can select a sub folder to have the plug-ins located in that sub folder displayed in the Tools Window.