There's a well-known problem in Excel 2008 (unlike Word 2008) where you as the user can't easily add the Subscript and Superscript formatting buttons to a toolbar -- these buttons appear in the Formatting Palette, but the commands don't appear in the Customize menu, meaning there's no apparent way to add them to a toolbar.
In Excel 2004, the same problem existed, but there's a Visual Basic for Applications workaround to add the buttons to the toolbar. Using that information, I created small AppleScript to add the needed buttons to the standard Formatting toolbar in Excel 2008:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080708090922927