plugin

You can´t find plugin for script-fu? What do You mean?
Plugins and scripts are 2 different things. There is no such thing as a plugin for script-fu.
Scrip-fus are .scm files, plugins are .py python files or other executables.

On OSX You need to set (almost always) executive rights to the .py and .scm files. If You don´t know how to do it using terminal, You can do it with BatchMod Tool. Search google or download it from Macupdate.
In finder, (Lion+) keep option (Alt) pressed down. Select Go from tab and there is your library. Inside the GIMP folder are plugins and scripts folders. Drop the scm files in scripts and python .py files in plugins folder.

You could also put them in the GIMP.app, located in your Applications. Right-click GIMP.app and select show contents. There are the plugins and script folders too.
But I don´t recommend putting them there. It´s better to use your library folder.

What comes to plugins and scripts, there are few that are windows only.
There are also at least 4 Native GIMPs for Mac. The official comes without additional plugins.
Lisanet Gimp and Partha have some additional plugins and scripts included:
http://gimp.lisanet.de/Website/Download.html
http://www.partha.com/