In Speedy entry, if you place the cursor on a note and press "P", an accidental will appear in parentheses. Press P again and the parens disappear and the accidental remains.

Another option in Speedy is to put the cursor on a note and press "shift-8". The accidental will appear. Press "shift-8" again and the accidental will go away.

Jeffrey,  Thanks for the suggestions.  Unfortunately, I tried them both and I get the same result.  It's if Finale doesn't recognize that there already is a Bb in the key signature.  If it doesn't insert a grace note, it puts in a flat.  At least it doesn't cram all the notes on top of each other and the end of the measure.

In Simple, select the note, and tap the N key. Or, if there is a Bb in the key signature, select the note, and tap the + sign on the numberpad.

Mike, I just tried both of them, and got the same result.  Could I have checked a wrong box when I first selected the key signature of D Minor?

I don’t know. Can you send me the Finale file? My email is in my signature.

Does the natural sign work on other documents? Are you using the Laptop Shortcut set, or the Standard set? Have you restarted the computer? Recreated the preference files?

Follow up: corruption in the score. XML Export/Import cleared it up.