Here's how I do it:

1. Choose the Articulation Tool and click on the note you want to add the flat/sharp over or under.

2. In the Articulation Selection box, choose Create.

3. Click on the box Main.... Then find the symbol you want. #98 for example is a flat sign. Click on the symbol you want and hit Select.

4. I do the same thing for Flipped...

5. Hit Ok. Now that articulation symbol is set to appear above or below a note.

6. Hit Select and the symbol will appear.

Are you doing figured bass? 

https://usermanuals.finalemusic.com/Finale2012Win/Content/Finale/Figured_bass.htm

Thanks to Martin. Exelent!