Originally Posted by Charles Kenyon

There IS "a simple menu item that allows new words to be added to the dictionary." The screenshot below is from Word 2013.

I already pointed out that the menu you are seeing seems to be the one for a grammar problem. It is possible to have both a grammar problem and a spelling problem with the same word. I do not know what happens then.

I again ask you to type "mistaek" in a blank document and not allow it to AutoCorrect. Then right-click on it. What menu do you get? I am asking you to try this to see if something is broken in your installation of Word or if what you observed was a conflation with a grammar error.

This is a peer-to-peer, user-to-user, support forum. I am not a Microsoft employee; no one here is. I am one of your fellow users of Word trying to help you solve a problem.