I have been trying to add ckeditor to Moodle 2.9, but I don't think this question is specific to ckeditor. I think it would apply to any third-party text editor:
After installing the ckeditor software to lib/editor/ckeditor, I added the following line to config.php in the root moodle directory:
When I navigate to Admin > Plugins > Text editors > Manage editors, I only see the original 3 editors: TinyMCS HTML editor, Plain text area, and Atto HTML editor.
FWIW, when I experimentally removed "tinymce," from the shown line of config.php, it still showed up on the "Manage editors" page. That makes me think that $CFG->texteditors is not being used. I do know it is being read, because I get an error on the "Manage editors" page if I put a syntax error into that line.
Thank you for any advice.