Plugging FCKEditor into Mephisto is very easy.
Step 1:
ruby script/plugin install svn://rubyforge.org//var/svn/fckeditorp/trunk/fckeditor
Step 2: Put the following in the application.rhtml file.
<%= javascript_include_tag 'fckeditor/fckeditor' %>
Step 3: Edit your _from.rhtml for articles. Replace
<%= form.text_area :body, :class => 'fat', :rows => 25 %>
with the following:
<%= fckeditor_textarea( 'article', 'body') %>
and place it inside a div tag and give a desired height value for the text area.
For more information visit: FCKEditor Plugin for Rails
Reference:
http://www.circlingminds.com/advanced-rails