System Preferences > Language and Text > Input Sources tab. Top of the left column, tick (check) "Keyboard and Character Viewer", Bottom of window next to search field, tick "Show input menu in menu bar"
System Preferences > Keyboard > Keyboard tab; tick "show keyboard and character viewers in menu bar"
For most purposes there's no need to use character viewer or change input source. All the standard diacritical marks and common symbols can be accessed directly from the standard keyboard. ü is produced by pressing alt (option) + U, release them and type U. Similarly é is alt+e, followed by e.
If you open Keyboard Viewer and press the alt (option) key, the location of the marks will be revealed. Note that five of them are highlighted in orange; these are the ones which can be used with multiple letters (acute, umlaut, gràve, circumflex and tilde)
Shift+Alt will show more.