Please remember that these scripts are provided for your use and education but are not supported by the Zenphoto team. This is true even if a team member originally created them. They may not work correctly with current Zenphoto versions.
Plugins are folders found in the "plugins" folder. They would be placed in the "plugins" folder of your installation. Note that what should be copied is the content of the folder. This will typically be a PHP script and a like-named folder with other content. The PHP script needs to be in the root of the "plugins" folder.
These plugins are also listed on our site's themes section: http://www.zenphoto.org/news/category/extensions
The folders in the "themes" folder are third party themes that are not hosted by the theme developer. Place the folder and its contents into your installation "themes" folder.
These themes are also listed on our site's themes section: http://www.zenphoto.org/theme/
You will find miscellaneous other extensions that do not exactly fit the above categories. Readme files or script comments will describe their use.