Logic's De-esser is completely useless. Seriously... don't waste anymore time trying to get it to function. Out of all the wonderful plug-ins we do get with Logic, that one is a complete and utter waste of code. I am not being tongue & cheek either...
The best software De-esser I have used is the UAD-1, but unless you have the UAD-1 card, it isn't an option. Sony (sonnivox, sorry) has one that's supposed to be really good.
Waves Renn De-esser does an admirable job as well.
There's also this new one from eiosis. I've heard favorable reviews on this one, and there is a downloadable demo.
http://www.eiosis.com/e2deesser
You could always try side chaining a channel EQ to a compressor, with a very narrow notch at the sibliant frequencies.
And lastly, while de-essers will help, you will get considerably better results by simply taking the time to automate the volume of the "Sss" down using volume Automation. It's time consuming, but ultimately sounds the best, and most natural.