Harmoniac is a physical model reed, wich can give harmonica sounds.
The VSTI version works as a simple synth. You load it in your favourite sequencer and feed it with midi notes.
The VST version uses audio input to feed the envelope follower. This gives you a much more natural control over sound. But you still need to feed it with midi to play a melody. If you blow or sing without hitting a note on your keyboard, it won’t give any sound. If you hit keys but don’t blow, it won’t sound either.