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So the advantage of hooking it up through the back is that 1) it's cleaner in my studio space. 2) it leaves the other inputs available for use.

I'm just curious because the Mtrack quad clearly has a diagram with a compressor plugged into the back... I'm just really confused why that diagram would show that if it's not possible...

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Maybe I missed it above? Any idea how to do it?

Use the inserts and compress going in and out. Control the effect via the hardware _ you should be able to null or bypass. I suppose you’ll have to use 3/4 for monitoring so you can hear the comps effect on the source.

Ok Thanks Mr. Happy and AT...  
 
I'm not using TRS cables - just TS cables. Does that have something to do with it? I'll buy some TRS cables...
 
Here's what I've done (I'm going to be overly explicit here) - which I think is what you both are saying - I've got a project open in Sonar with a mono guitar part that was recorded with a mic to an amp through a preamp into the M-track.
 
I've plugged a TS cable into the output 3 of the M-Track (output 1 and 2 say "monitors" and are plugged into my monitors). That cable goes to the "in" of the RNC under the section labeled "Left (or mono)".
 
I've plugged a TS cable from the "out" of the RNC (Left side) and plugged it into the M-Track "Insert 1"
 
The I go into Sonar to the effects bin - "Insert External Insert" - I put the Send at "Driver 3" Mono and - here's where the problem is - On the return menu, I only see the front drivers in the dropdown menu... 
 
Does that make sense? What am I doing wrong?!

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OK - I think I'm getting some clarity of the problem here...  I tried it with the m-tracks 1,2,3,4 input and it works.
 
I would like to pursue the idea of using the hardware "inserts" on the m-track and RNC. Can you help me through that? 
 
I'm going to buy a "Y" TRS cable... Is that the missing link? Is there something else about those insert connectors that I'm missing?

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Another clarifying question - are those inserts in the back only able to be used at the recording stage? Meaning before the signal even goes into sonar? I was assuming that I could plug it in there, record a dry signal in sonar, and then play with the parameters after recording - perhaps I'm going about this all wrong and I should focus on the signal before it reaches Sonar?