A little bit of a puzzling one, and not appearing to be much information about this, seemingly straight-forward task. Enabling the spellchecker plugin in TinyMCE in Magento’s admin. Here’s how you do it;
In js/mage/adminhtml/wysiwyg/tiny_mce/setup.js, find “theme_advanced_buttons3”, and update it as follows;
theme_advanced_buttons3 : 'tablecontrols,|,hr,removeformat,visualaid,|,sub,sup,|,charmap,iespell,media,advhr,|,ltr,rtl,|,fullscreen,|,spellchecker',
Then find “var plugins” (should be a handful of lines above the one you just changed), and update it as follows (this one tripped me up, as not doing this will simply tell TinyMCE to ignore that “spellchecker” button you just added).
var plugins = 'safari,pagebreak,style,layer,table,advhr,advimage,emotions,iespell,media,searchreplace,contextmenu,paste,directionality,fullscreen,spellchecker,noneditable,visualchars,nonbreaking,xhtmlxtras';
Finally, download PHP Spellchecker from TinyMCE’s website, and extract into js/tiny_mce/plugins/.
Reload the admin, open a WYSIWYG editor, and there it is, at the end of the third line of icons!