This might be very late, but I've just found how to type ü and ß on a Mac.
Quote:
On a Mac:
Use the OPTION key. Hold down OPTION and push ‘s’ to get ß. For the umlauted characters, hold down OPTION and push ‘u’. Release OPTION, then type the desired base letter (a, o, u, A, O, or U). The umlaut will appear over the letter you typed. (So to type ü, you should hold down OPTION, press u, then release OPTION and press u again.)
