A family of Microsoft word processing software products for creating web, email, and print documents.
Make it easy on yourself 🙂:
If the word is marked as misspelled in the document, right-click it & select Add to Dictionary in the contextual menu.
Also, when you run the Editor any words suspected of being misspelled will appear in the Editor Task Pane. Click Spelling in the Editor Pane then use the Add to Dictionary command in the lower portion of the pane.
If it is recognized as correctly spelled [unmarked] there is no need to add it to a Custom Dictionary because it already is in the Main Dictionary. Opening the Custom Dictionary for editing should be necessary only if you need to add a collection of terms or need to remove a term previously added.
One other consideration is the Language attribute of the term but that is a separate issue.