Registered as admin?
Added to MIDI Track FX bin (it's an MFX)?
Worked fine for me on X3 after doing that (I tried feeding the MIDI track's output to PSyn just because it was easy) .
UPDATE: The only oddity I've found (annoying) is that on closing the plug-in's window and re-opening, the settings appear to have gone, even though they seem to take effect on playback. I worked round this by saving as a preset and reloading it if I needed to work on the settings.