How do I use/configure the spellchecker?

In Explorer, you've got two buttons for spellchecking with the dialog, or with the Active Spell.
In Classic, you don't have the toolbar, so you can right-click within the clip, and access the spell checker that way.

There is also a Thesaurus, if you've installed it. Right-click on a word in the text editor, and you can access it from there.

Complete information on configuring the spellchecker, installing dictionaries and the Thesaurus, is here.