How do you write a double sharp or double flat in Finale Notepad?

just use the +halfstep or -halfstep tool.
( not available in all Finale products)

tip. I might not be entirely right, but in Finale can't you just go down a note and make that a sharp instead of the needed a double flat.

you can also upgrade to Printmusic or some other Finale product.

Its not very expensive and the quality is amazing.

Ctrl + click the note. Click the "Simple" toolbar, right next to Trade-Up. Go to "Simple Edit Commands," "Accidentals," and then click "Double Flat" or "Double Sharp." Refer to the Composition Program Help in the Composition forum in the future...