Not an intuitive plug-in, lol I'm trying to morph from one sound to another. I need a breakdown of the processes involved. The Melda help tells you how to use the tools but not how the tools work, if that makes sense...

You will want to be using the VST 3 version - side-chaining is what it's all about, and that's something between difficult and maddeningly difficult with VST 2.
 
The plug-in needs two inputs: that of the track it's inserted into and the side-chain signal.  The ratio control mixes the two signals according to the ratio control setting.

It is a little difficult to get your head around at first. You need 3 tracks, source 1, source 2 and combo. First put MRatio on combo. Next route source 1's output to combo. Go to source 2 and set its sidechain for MRatio. After that is done set the sidechain to post and turn the volume all the way down on source 2. Finally go to MRatio and use the mix slider to adjust the ratio between the 2 signals.

Here is a video I did a while back showing how to use it, but keep in mind this is about parallel processing, so it is a bit different because I'm only using 1 signal. Hopefully it will show you how to set it up in Sonar though.
https://youtu.be/efP4-hiMOeQ

Chandler
It is a little difficult to get your head around at first. You need 3 tracks, source 1, source 2 and combo. First put MRatio on combo. Next route source 1's output to combo. Go to source 2 and set its sidechain for MRatio. After that is done set the sidechain to post and turn the volume all the way down on source 2. Finally go to MRatio and use the mix slider to adjust the ratio between the 2 signals.

Here is a video I did a while back showing how to use it, but keep in mind this is about parallel processing, so it is a bit different because I'm only using 1 signal. Hopefully it will show you how to set it up in Sonar though.
https://youtu.be/efP4-hiMOeQ

Riddle me this - In the intro of the video there are drums, which change from clean to effected gradually; are you manually changing the ratio as it plays or has it been programmed to change gradually over time in the song automatically? Can I automate the mix slider in MRatio?
 Thanks a million!

Ha, figgered it out - Just automate the ratio!
 
Thanks, Chandler!