You have to talk to the FCKeditor guys about that. On their website they mention IEspell, but this works only for MS Internet Explorer. On their demo site, I see the spell check icon, but it's greyed out. In this forum, spell checking works for me using Firefox 2.0 under Windows after I installed the Firefox extension "United States English Dictionary" from here. Maybe this works for you also under Linux. There is however one problem: I get wrong words underlined in red, but I cannot get suggestions by clicking at the wrong words. For FCKeditro version 2.6+ however it was announced that there will be a new spell checker, so maybe you have to wait for their update.