Check that the Number Lock is on. (It usually is by default). Hold down the Alt key. Type 171 on the numeric keypad, NOT on the numbered keys at the top of your keyboard. Release the Alt key.
If you're using a computer without a numeric keypad (such as a laptop), look for the numbers in colour on lettered keys and access them by holding down the Fn key (as well as the Alt key).
Lots more special characters, such as ¼, ▲ and ♂ here:
http://www.tedmontgomery.com/tutorial/altchrc.html