Well, if your german keyboard is anything like my swedish one (at least we have 'ä' and 'ö' in common), you should find them by typing Shift+Alt+8 and Shift+Alt+9. If not, you can always use that old time favourite; KeyCaps. That program is located in /Applications/Utilities/. It's great for finding out where all those little special characters have been hidden.
Good luck!