Umlaut?

you can find the ones you want by looking in the special characters entry in the edit menu

And for these you depress option and they the character (sometimes followed by typing the character again without the option key - many have keyboard shortcuts

å é î ø ü

LN

You can do it in two ways. one press option+u, release option and type a vowel. for example option+u, release option+o gives ö .
Alternatively you can go to international system preferences->input menu. check german and check the check box to show it in the menu bar. you can then switch input menu to alternate between english and german keyboard. This is ü typed using german keyboard layout. You can also check keyboard viewer in the same system preferences pane. it will then be available from the same input menu in the menu bar.