On this page http://ckeditor.com/addon/youtube it says:
- Enable the plugin by changing or adding the extraPlugins line in your configuration (config.js): config.extraPlugins = 'youtube';
...this didn't work (YouTube link didn't show up) and I found out somehwere else that actually you have to add this line of code to ckeditor.config.js - after this the Youtube icon show up.
If you click the Download
If you click the Download button here at the top and look at the contents, you'll find that the configuration file is named config.js, there's no such ckeditor.config.js so if your installation is using that file is because you're using it integrated in some CMS that has changed the default value and you can't blame the plugin author for not testing with every possible CMS out there to find out what are the steps to add his plugin there.