Shouldn't need the fn key. All the common symbols and diacritics are available using alt (option) or shift-alt.
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"
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.
To use the highlighted ones, type alt+key, followed by the letter the diacritic is to go over.
For example, alt+e followed by e = é, alt+u followed by u = ü, alt+n followed by n = ñ.