I'm going to assume that you have Mac OS X for running iMovie. If this is the case, your plug-ins should be placed in the following place:
Hard Drive -> Users -> (User Name) -> Library -> iMovie -> Plug-ins
If you are running OS 9, then your plug-ins should be placed here:
Hard Drive -> Applications (OS 9) -> iMovie 2 -> iMovie -> Resources -> Plugins -> Then the Respective type of plugin.
Hope this helps. 😀