In a Word doc, you would need to install on your computer a special font that includes music notation symbols; sharps and flats are not included in standard fonts. I just use # (number sign) and b (lower-case B) in my documents.
In Set List Maker, there is no way to install extra fonts, so no way that I know of to type a real flat symbol. But you can use # and b just by typing them on the on-screen keyboard.