I'll try to remember what I did...

a) You need the spell.js, spell.css, SpellCheck.aspx, and dictionary files from the spell checker in your .NET app. I created a directory called spell and put them there.

b) Pages using the spell checker need to include the spell.js Javascript code.

c) Finally, I added a spell check button to the editor that calls (note the use of my spell directory in the path):

 checkSpelling(controlToCheck, 'spell/SpellCheck.aspx');