Audacity uses the "VST Bridge" to support VST plug-ins. Unfortunately, due to licensing restrictions, the Graphical User Interface (GUI) is not supported. This is why you just have a bunch of sliders instead of a pretty and intuitive GUI.
To use the Spitfish De-esser you may need to find a manual for it, and see if you can match up the names of the sliders, to the controls in the GUI.
I am working on a Nyquist de-esser plug-in. Progress so far is good, and I am hoping to release it for Christmas (subject to having the time to get it finished. If you would like to PM me your e-mail address, I'll e-mail you when it is available and you can help me test it.
To send a PM (private message), click on the PM button below my name.
A bit about De-Essers:
They are not magic.
Basically, they suppress high frequencies (within a selected frequency band) if they exceed a selected threshold level.
With normal voice recording, there will be fairly high frequencies present throughout the speaking/singing, but sometimes certain high frequencies may be over emphasised. This may be due to a poor recording, or simply because of the persons voice (whistling through false teeth, for an extreme example).
These exaggerated frequencies are most common on "sss" sounds (sibilance), but may also be present on other vocal sounds such as "T" sounds. A de-esser is not able to distinguish whether the sounds come from a "T" or an "sss", so De-essing may have an unwanted side-effect of also reducing the definition of sounds other than "sss's".
As with all "corrective" post-production processing, there is a cost in sound quality, so it is usually best to start with the best, cleanest recording possible, and only try to "correct" the audio as a last resort.