right, it depends how you want to use the RNC, i.e for mixing or for recording?
for recording, you'd need to get an external pre amp and go;
Mic ---> external pre ----> RNC -----> Mbox Line in -----> Logic
if you want to use it for mixing, here's what i do;
Set the output on the track i want to compress to "3-4", pan it hard left (so that the signal only goes to output 3) and put the fader to 0db. then, on the mbox;
Line out 3 ----> RNC in then RNC out ----> line in on Mbox. create a new track in logic, set the input to whichever line in the RNC is plugged into and arm the track to hear it.
this means you can monitor the sound of the compressor separately to your mix and then listen to it in the mix as well. if you just want to hear what you're compressing, mute the "1-2" output on the far right of the mixer. to hear it all just unmute it. when you're happy with it you need to record the track back in to Logic. bare in mind this will add some latency so once you've recorded it back in to logic you'll need to drag the track back a smidge in the arrange window (i normally create the new track below the one i want to compress so that when you come to move the newly recorded track you can line it up with the track above perfectly)
Also, as everyone else has said, 10:1 compression is crazy, especially on the RNC. at most i go for 6:1 and that for peak taming. tbf, the manual does say, if you've not used compressors before, start by putting all the knobs to their "12 o'clock" position and then adjust from there. also, the "super nice" mode is perfect for smooth sounding vocal compression so try it