Here is a link with directions on how to add the umlaut for German or the tilde for Spanish.
“You can use Keyboard Viewer to see and type characters used in alternate languages other than your chosen language.”
http://docs.info.apple.com/article.html?path=Mac/10.5/en/9024.html
If you use Keyboard Viewer, you can see the keyboard layout on your screen.
To access Keyboard Viewer:
Choose Apple menu > System Preferences and click International.
Click Input Menu, then select the checkbox next to Keyboard Viewer.
Select the On checkbox next to the language whose keyboard layout you want to use.
Choose Show Keyboard Viewer from the Input menu (international symbol) on the right side of the menu bar.
äöü
Mac OS X 10.6 Help
To turn on the Keyboard Viewer:
Choose Apple menu > System Preferences, click Language & Text, and then click Input Sources.
Open the Input Sources pane of Language & Text preferences
Select the Keyboard & Character Viewer checkbox.
You can very easily use diacritics in Mac OS. Use the Option key.
Option-u, for example, gives you the umlaut like this ü, ï, ö, etc. Hold down the Option key, press down the letter U, and then type the letter you wish to put the umlaut over. With Keyboard Viewer open, hold down the Option key to see the available diacritics and other symbols.
Examples:
Option-s produces the eszett. Straße.
Option-c produces the letter ç.
Option-n produces the tilde: ñ.
Option-a produces the letter å.
Option-o produces the letter ø.
And so on.
Don’t be afraid to experiment. Mac OS is very intuitive.