This feature is no longer supported or relevant because browsers now have spellchecking, and that's where it should be handled.
Spellchecking Admin
Each Tiki feature may have its own spellchecking option.
Articles
To enable spellechecking with creating or editing articles, select the Spellchecking option in the CMS Features section of the Articles Config page.
CMS Features section of the Articles: Config page.
Wiki
To enable spellechecking with creating or editing a wiki page, select the Spellchecking option in the Wiki Features section of the Wiki Config page:
Wiki Features section fo the Wiki: Config page.
I mportant note about installation
Spellcheck is not bundled. You must get the language file from Sourceforge and import the data with phpMyAdmin or similar.