"FindNextMisspelling" is a front end token that selects the next misspelled word and opens the spell check dialog box.
This token is equivalent to the menu command Edit Check Spelling.
When a misspelled word is found, it is displayed in the dialog box along with a list of suggested alternatives. Click a suggested word to copy it to the "Replace with:" field. Double-click a suggested word to paste it into the notebook and continue the search.
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].