First, open the Plug-ins window from the PlugSearch drop down menu in the Finder task bar. Then you can drag-and-drop Plug-ins from the list to each corresponding Favourites box. Here you can also reorder the favourites.
You can also mark Plug-Ins as favourite by clicking the yellow star button right of each Plug-In in the search results, or in the Plug-In Info Panel, which you can open by pressing the right arrow key.