The CDN link sometime goes down - it is not 100% reliable.
It should be straight forward to download the library from the like I gave above.
1. are you using running tiddlywiki under nodejs - a file url will not work in this case.
2. to check that the browser can see your ckeditor library try typing the full pathname of the library in the browser ie file:///root/node_modules/ckeditor4/ckeditor.js - you should see the js code appear.
Once you have your local version working the instructions to include an plugin are here:
The config is exposed in the tiddler
$:/plugins/bj/visualeditor/config.json