Setting up Sonar Platinum to compress master output with sidechain eq.

There are indeed many cat-skinning techniques. But each one yields different results.
 
Frequency-dependent compression can mean [at least] two different things: filtering the sidechain, or compressing specific frequency bands. Two very different effects, and I'm not sure which one you're after.
 
In the first method, the compressor acts normally (and can be either broadband or multi-band) but it's looking at a filtered sidechain when deciding how much to compress. That's normally done within the compressor, and requires that the plugin provide a sidechain filter. Many compressors do, although the sophistication of their filters varies. One plugin that's designed specifically for doing this on the master bus is the free TDR Kotelnikov bus compressor, although in the free version its sidechain filter is limited to high-pass. My favorite is Fabfilter's Pro-C, which offers a full parametric EQ for the sidechain filter.
 
In the second method, the main signal is split into two or more frequency bands, each of which gets its own compressor. The Sonitus Multiband works well for this, but you may find the newer LP-MB easier to use and potentially more transparent. 
 
There is a third option: dynamic equalization. My favorite tool for this is MDynamicEQ from Meldaproduction. Dynamic EQ avoids potential issues inherent in multiband compressors from splitting and recombining the signal, because it's an equalizer rather than a compressor. It's just as easy to use as a compressor, though.
 
Which of these methods one might choose depends on the desired effect. On the master bus, a common problem is when low frequencies dictate overall compression, which can, for example, cause a dominant kick to smack down an innocent tambourine. If that's your scenario, I'd recommend Cakewalk's LP-MB with a single band.