First pick an editor. PhpWebSite supports the following:

FCKeditor - http://www.fckeditor.net/
TinyMCE   - http://tinymce.moxiecode.com/

Get the tarball/zip and decompress them into the appropiate
javascript/editor directory.

To enable the wysiwyg editor go into your config/core/config.php file
and look for:

define('USE_WYSIWYG_EDITOR', FALSE);
define('DEFAULT_EDITOR_TOOL', '');

Set the first define to TRUE.
Set the second define to the name of the directory where your wysiwyg
is untarred/unzipped.

For more information, please read Editor.txt.