Web Site Publishing Pro plugins are installed using Lightroom's plugin manager. The plugin will be distributed as a zip file, and once you've unzipped it you should have a plugin folder named, for example, "ImpactPro.lrplugin" (the actual name will depend upon the plugin).
On a Mac this plugin will look like a single file and have a nice icon.
On a PC the plugin will be a folder containing lots of files — the folder itself is the plugin. You should make sure that the plugin folder hasn't been unzipped into a parent folder with the same name — if this happens, you must point to the child folder when using the Lightroom plugin manager (below).
The plugin is installed via the Lightroom plugin manager.