How to use NI's The Mouth with X1 or X2??

You insert it as an effect rather than a synth.

I know this is an older post.  I joined to help you out.  Markyzno is right.  The illustrated guide to help you is in NI's knowledge base.  Easy as pie.
 

 
Works beautifully along with automating Mouth controls while recording the audio or after. All recorded parameters show up in their own automation lane(s) as well.
 
In case the above link is forbidden here, here's the rundown.
1. Choose an audio track and insert Reaktor as a soft synth - NOT audio - into the FX bin.
2. Load an Ensemble (Mouth in your case) into the Reaktor GUI.
3. Choose either an existing MIDI track or create a new one.
4. Click the 'Output' at the bottom of the MIDI track and select Reaktor(1), (2), (3)... (whichever instance, if more than one Reaktor instance is loaded.
 
Your're now routed.
 
5. Now arm the MIDI track and as you record, the notes will control the Reaktor Ensemble. Alternately, just draw the notes into the MIDI track.
 
Best of luck. iGoNow

I'm really only aware of Tim Exile, the creator of The Mouth. He also created The Finger.  I can't link it here, but youtube "The Mouth - Tim Exile Performs LIVE" where you'll find other related vids, too.  I use it for ambient stuff, but haven't released any tracks yet - actually later this week.  It's quite a phenomenal piece of software and designed for live input, though can be used with fixed parameters as well and works flawlessly in Sonar.