Sorry about the deluge of info above, - sometimes too much information is as bad as too little, - and makes it look a lot more complicated than it is.
You only need to use the duplicate site definition that you have created when you need to adjust the CMS' HTML Editor configuration. With PowerCMS you only really need to do this once when you want to add your own styles from your site stylesheet to the PowerCMS HTML Editor.
The rest of the time, you you will use the original site definition for your site - (the one you made a duplicate of to point to just the CMS folder) - which shows all of your site's files and folders. Both site definitions use the same files, it's just that we needed to 'trick' Dreamweaver into thinking that the CMS folder was the root directory for a while.
Please post back with any questions or for clarification of any points.
It wouldn't do any good to add styles to the fck_editorarea.css, - that is an internal style sheet for the editor itself - not for styles that you can choose to insert into into your pages.