adding a FCKeditor toolbar button

Hi,
I'm trying to add a plugin, but it doesn't work!
The button doesn't appear in the toolbar (the message is:"unknown
element").
I've used tutorial like this one:

http://www.kindblad.com/2008/09/07/fckeditor-2x-how-to-add-a-new-button-with-custom-javascript-to-the-toolbar/

I've tested it on a sample plugin too. Same result!

Help me, please!