This isn't exactly the satisfying answer to your question, but in Behind Bars, Elaine Gould also cites using an open circle, like a harmonic-type symbol, an "o" attached to the end point of a decresc which acts as the same thing.

I am able to get the character function to work creating a new dynamic on a mac (also Finale 25).

You could always bold and italicize a Times New Roman n in a pinch (not ideal, I realize)... maybe re-install the font? I'm sure someone with more expertise in Windows can be more helpful.

In which font does ALT+0150 produce the symbol you want?

It is in the Maesto font.  I'm going to Help>Shortcuts and Character Maps>Maestro Character Map>Dynamics and that is where it says it should be Alt+0150.  

I found this using Windows' Character Map at code 0x96 and copied and pasted it into the expression designer.