As suggested above here is a workaround. Keep a copy of Notes open with a new 'document'. Every time you get a red squiggle in Evernote copy the word, paste it into Notes, and then when it shows a red squiggle in Notes just right click and add it to the dictionary. Then when you use that word in Evenote it is seen as correct. It seems obvious that Evernote is using the Mac dictionary but not permitting edits of it. This did not work when I tried to add a word when using MS Word.