- Press and hold 'option' + 'u'
(you should see ¨ appear with an underline) - Release both keys
(you should still see ¨ with an underline) - Press vowel you wish to have under the umlaut
(underline will be replaced with letter you have typed)
Ää, Öö, Üü
Having difficulty? Post a comment below.
Source: How to use an Umlaut on a Mac (eHow.com)