You've missed it; the ñ character is ASCII 241 or Unicode 0F1 and is
present in most fonts. In the Insert > Symbol dialog, with "(normal
text)" selected in the font box and "ASCII (decimal)" selected in the
dropdown at the bottom right, you'll find the character 15 characters
from the end of the list.
The default shortcut for it is to press and release Ctrl and the
tilde key together, then press the n key.
Or you can hold the Alt key while you type 0241 on the number pad
(with NumLock turned on).
Or (in Word 2003 or later) you can type F1 in the document and press
Alt+X.
See
http://sbarnhill.mvps.org/WordFAQs/InsertSpecChars.htm for more.