I'm having a shot at mastering a track. I've got a stereo mix in Track 1 and I've done the low and high cut. 

I've inserted the Sonitus Multiband Compressor. When playing the track, each band is showing about -18 on the meters, a little less in the bass. Questions: 
 
1) What am I trying to achieve here?
I read that the objective is to get wayward peaks under control and compress the sum of all bands but I don't see anything spiking, peaking or misbehaving and I don't see a 'master' fader or any way of compressing the sum of all bands.  

Unlike the 2 band version, I don't see a knee or compression display when playing with the Threshold faders or adjusting those controls - what's with that?

2) How do I know if any of the bands need compressing?

If anyone has a tut for this thing, please let me know - or if there's a better tool.

Thanks - rev

Hey rev,
The Sonitus multiband compressor doesn't compress the sum of all bands. It allows you to focus on specific frequency ranges that may need attention. There is no master fader.
Multiband compressors are good when you don't want to apply the same compression to the whole frequency spectrum.
The help file has a lot of detailed information, and will tell you much more than I can.
Another multi that I like is the GVST GMulti. It has very different graphics, but is more intuitive for me.
And, as always, I keep Voxengo SPAN in my master bus to keep an eye on my final output.
You may not even need to use a multi, but if you do, use plenty of discretion, as you can quickly get into trouble.
Feel free to experiment, but do lots of save as, in case you mangle your mix.

Bob:

Thanks for that! I didn't know there was a Help file for the Sonitus stuff - found it now. I think I want something that tells me when a frequency range needs attention, the way VU meters turn red past a certain point. Without something to indicate a problem, I don't think I'd know it was there.

Experiments with Boost 11 told me that it's changing my mix and in a way I couldn't control. I'm going to check out the others you recommended. 

Thanks a lot!

Try the LP 64 I think it's a way better multi band, and it has an input gain. If your tracks are below the threshold then the meters will not show action. If only the low end is compressing then that's telling you you have to much and need to back off but at the track level, not by cutting that band. I find it's a good way of balancing frequencies in the mix. I try and have all bands just flickering the meters. 

What you want to experiment with is putting the multiband compressor in front of your master limiter (and after your master bus EQ if you're using one). Used with gentle ratios, this will bring up your overall volume in an even-handed manner rather than letting the limiter do all the maximizing work. Most people use multibands to achieve very loud, pumping mixes, but I use it for just the opposite effect: transparent leveling.

The Sonitus multiband is pretty good for this, although I wish it would let you specify the number of bands, because often you only need two, and the Sonitus has a fixed number of bands.

Here's how I initialize the Sonitus Multiband for mastering. I start on the Common tab and set all 5 bands to a ratio of 2:1 or 3:1, a very gentle slope of around 19, attacks at about 20ms, release between 40ms and 150ms. I set band 1's threshold at some value where it's just showing some compression, then set each subsequent band's threshold about 3db lower than the one to its left. Then I start tweaking, first pulling thresholds down as a group (maintaining the ~3db ratios), then adjusting the makeup gain.