"It is actually easier than that.

Do a Get Info in Finder on iPhoto. Choose Plugins from the menu. There's a
large Add... Button in that panel. Use it to navigate to your new plugin. It
will be copied to iPhoto/Contents/Plugins.

The interface (like much of Mac OS X) isn't finished yet. You can pick
absolutely anything (it doesn't have to be a plugin) and it will be copied
to the plugins folder. Also, the list of plugins isn't updated immediately,
you may need to Get Info on a different file and then come back to iPhoto to
see your newly-installed plugin.

If you disable a plugin it is moved to iPhoto/Contents/Plugins (Disabled)."