Key codes for special characters in German

On a PC:

Make sure the NumLock key is on.
While pressing Alt , type the digits on the key pad:

Alt + 132 = ä
Alt + 142 = Ä
Alt + 148 = ö
Alt + 153 = Ö
Alt + 129 = ü
Alt + 154 = Ü
Alt + 225 = ß

On a MAC:

Umlauts: Press the option key + u. The next letter pressed will have an umlaut.
ß: Press the option key + s.