One more:
Side chaining in FL Studio 91. Route (for instance) bass to first mixer track and kick to second.
2. Select 2nd mixer track, then right-click small gray arrow at the first track. Choose "SideChain to this track"

3. Select 1st mixer track (with bass on it). Add Fruity Limiter there, a) switch to compressor b) select side chain source c)adjust threshold, ratio etc. It should be working now and look like:
Side chain VST compressors (and other multi audio input plugins) are also can be used. You should go to its settings and map audio inputs. In case of Density mk II "auto map" worked just fine.
