First make sure the numlock light is on on the keyboard (it normally is), if not press the Numlock key on the number pad. Use the digits on the number pad for codes, not the querty part.
For a flat note type 266d and then press Alt-X, the 266d you typed changs into a flat sign. (these are unicode characters and will work in Word, but not here).
A natural note is 266e and then press Alt-X and a sharp note 266f and then press Alt-X
For the popular ♪ sign you can hold the Alt key down and press 1 on the number pad.
The ♫ comes up if with Alt+14.