Hi,
The documentation about FCKeditor spell cheking is here: http://docs.fckeditor.net/FCKeditor_2.x/Developers_Guide/Configuration/Spell_Checker
If you are using Firefox, you can look in /ClientScript/mojofckconfig.js and set this to true:
FCKConfig.FirefoxSpellChecker = true ;
I don't know how that got set to false as it worked before. In any case the only thing this does in Firefox is put a dotted line under words that it thinks are mispelled, it doesn't seem to offer any suggestions as far as I can tell.
For IE, look for the toolbars in mojofckconfig.js, in the one named "Full" add this to the first row right after 'Print','SpellCheck'
I just did the same thing to my copy so it will be enabled in the next release. Not sure how or when those settings got lost.
The check spelling icon will appear right next to the print icon, but only in IE.
Hope it helps,
Joe