howto add a sharp on a mordent?

I don't know if this is the best way, but you can create a text element (eg, ctrl-T for staff text), then use the F2 text symbol palette to enter the sharp sign. This then allows you to size it as desired. There is also the regular symbol palette (Z) from which you can drag symbols to the score, but those can't be sized s far as I know, and the regular sharp sign is bigger than you'd want in a mordent.