Are you talking about the Character Palette?
Activate it in System Preferences > International > Input Menu.
How about this then: http://www.macosxhints.com/article.php?story=20071123103246253
For some unknown reason, the international data caches can get corrupted and prevent Character Palette and Keyboard Viewer from appearing on the Input tab of the International System Preferences pane. This also breaks the Special Characters entry in the Edit menu of many applications, including the Finder, Safari, and Text Edit.
If this happens to you, go to System/Library/Caches folder and trash the following files (don't worry, they will be re-created when you restart your Mac):