How does the pronunciation feature work?

I have long wanted a way to fix those annoying mispronunciations of the Mac text-to-speech feature, so I was glad to read somewhere that you can go to System Preferences > Universal Access > Seeing tab > Open Voiceover Utility > Pronunciation tab, and then you can tell the Mac how to pronounce certain words. I added the words I wanted, used the "Play" button to make sure it pronounced the words right, and saved my changes, but when I go to an application that uses TTS such as TextEdit, it still pronounces the words wrong. How does this work? Thanks.