This addon is a plugin. You need to extract it to your /plugins folder to install it.
This is a what-you-see-is-what-you-get plugin for Vanilla. Unlike TinyMCE / FCKEditor, this one is very very small, uses jQuery, and still provides very good features. It works in IE6, IE7, IE8, FF2+, Safari3+ and gracefully degrades to the previous textarea in other browser. It is safe contained (no need to download anything else).