ñ is not in the insert Special Character
Try changing to a different Font on Special Character dialog screen. I found it on various fonts including: Courier 10 Pitch, Bitstream Character, and others.
Insert/ special character
It’s there in the Latin 1 subset. You may need to scroll down.
If you are using Windows en-US install the US-International Keyboard (look Cotrol Panel, Languages). Then you can type ~+n to get ñ or AltGr(right Alt-key)+n. You have also many accented, umlaut characters available.