1/2.  How do I use instruments from the [Instruments] menu? Or are
these used internally by LMMS?

I think some instruments don't have GUI's, you should control them through MIDI.

3/4. Why are there items under the Unavailable Effects list?  Are
these effects un-developed?

AFAIK unavailable plugins are not loaded by LMMS 'cause it doesn't support connecting other ports than the stereo ones to a plugin. One example is SC3 (sidechaining compressor). It has 3 inputs (1 for SC reference and two for the actual audio stereo input) and 2 outputs (stereo). LMMS cannot handle this third port yet, it just connects the two inputs and two outputs to the FX channel where you loaded the plugin (it actually won't load a plugin with 3 inputs and place it into the unavailable plugins folder).

Rodrigo Rodrigues da Silva
Engenharia de Computação - Coop8

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