Why can't I get Sonitus Compressor to sidechain? I do the following steps:
1) Insert Sonitus compressor into audio track FX Bin
2) Click on Output and click on 'Selected Track Outputs....'
3) On Audio Outputs dropdown select 'Sonitus: fx Compressor(side input)- Track (X)
4) All I ever get is this:   (tried to attach snapshot image)
    Pop-up window says "The following Track(s) can not be routed to Sonitus:fxCompressor (side input):    Track (X) 
                                    Would you like to continue?"
 
Even if I click 'Yes', nothing happens. What am I missing? What am I doing wrong?    Please HELP !!!

Hang on, are you using the audio output of the same track you're using the compressor on as side chain source? It seems obvious that that won't work. Unfortunately your image doesn't show, you'll have to upload it somewhere (imgur/flickr) first.

For a simple dance ducking example, you'd put the sonitus on, say, a pad sound, then use a kickdrum track and create a send on it to the sonitus side chain input.

Actually, trying to create Craig Anterton's Tip#69 - "Lifter"  (an FX Chain)

   In order to create this "Lifter" FX Chain (which is two compressors), I should just create it and leave the UI Audio button as is (grayed out), and when I apply it to another track's output, then the Audio/Sidechain button will become enabled?
   I tried this and it works as above on a Synth's Track output, but not on an Audio Track's output. This I don't understand. (then again, I'm a little sleep deprived right now)

I am completely failing to understand the steps you described BUT two simple problems come to mind...
 
1) As Sanderx pointed out you may not be routing it properly. The compressor goes in the FX bin of the track to be affected (such as ducking). It needs source audio from ANOTHER audio output (that goes into the Sonitus sidechain input) such as a kick drum to make the compressor trigger thus affecting the track it resides on.
 
2) If that routing is properly set up then you need to make sure the compressor settings are set to actually do something when the trigger signal is received. In the example of the "duck" method if the source signal level (such as a kick) is below the compressors "threshold" the compressor does nothing. Bring down the threshold to below the peaks of the trigger signal then the compressor will start doing something to the trakc hosting the compressor.
 
I recall perusing the "lifter" post breifly but can't remember the exact effect described but yeah... for sidechain stuff you have to make sure a) the routing is correct and b) the compressor/gate/whatever settings are setup to actually DO something otherwise no effect occurs.
 
Also... unless it's a really percussive sound or something with lots of spikes it's possible the compressor IS being trigger but it's not noticeable. Like trying to use a fairly steady guitar track to duck another guitar track or a drum track or something would just apply general compression whereas a kick or snare will smash the compressor for an instant then release (with the right A/R settings) thus causing a noticeable momentary change in the volume level.

The Lifter isn't intended to be used with a sidechain signal...but if you come up with anything interesting from doing so, let us know...

   OK - Here's the deal: The compressors in this FX Chain are not supposed to be sidechained. It's just that the image that was posted in the (C.A.) Tip #69 shows them with sidechain, and the instructions said to set-up the compressors as shown in the image. (the sidechain button shown was disabled, it just happened to be that way)
   Sorry forumers, I didn't eat my Wheaties today