The built-in spell checker in OS X is quite handy and available from within all Mac apps including DEVONthink. But there are many times when it thinks you’ve misspelled something when you know you haven’t. And while it may be nothing but a cosmetic annoyance to see the dashed red line under a word, you may want to teach the spell checker a thing or two.
Highlight the term that’s marked as misspelled and right-click (or Control-click) and choose Learn Spelling. If you learn that you were indeed wrong in your spelling you can highlight the word and choose Unlearn Spelling.