I have a textarea with CKEditor (bbCode Plugin).
You can't simply add text to the CKEDITOR via jQuery, instead go with api given by CKEDITOR
CKEDITOR.instances.editor1.setData(data.data.description);
Here your code looks like
$.getJSON('http://gdata.youtube.com/feeds/api/videos/'+video_id+'?v=2&alt=jsonc',function(data,status,xhr){
CKEDITOR.instances.editor1.setData(data.data.description);
});
Fiddle
Instead of writing the description directly into the text area, try the CKEditor setData method. You can find a description of it here:
http://docs.ckeditor.com/#!/api/CKEDITOR.editor-method-popup
also be sure that your description variable does have a value, I'd use a temporary alert(description);
for this but you may be able to do it with a javascript debugger also.