If CKEditor does not appear on the page, check if all files were extracted correctly.
The sites/all/modules/ckeditor/ckeditor/ directory should contain at least the following files: ckeditor.js, config.js, contents.css and directories named images, lang, plugins, skins, and themes.
The correct directory structure is as follows:
modules <dir>
ckeditor <dir>
ckeditor.css
ckeditor.info
...
ckeditor <dir>
_source <dir>
images <dir>
lang <dir>
plugins <dir>
skins <dir>
themes <dir>
COPY_HERE.txt
ckeditor.js
...
As noted above, alternatively the sites/all/libraries/ckeditor directory can be used. The libraries directory is the default path when drush is used to download the editor JavaScript code.
If you are still experiencing problems with your CKEditor installation, see the Help & Contribution section.