I’ve just installed Ardour 4.4.0 under OSX 10.9.5 - however when i create a MIDI track, insert an AU and click it in the mixer no GUI is visible, just a grey screen. The plugin plays from the sequencer timeline…but I can’t bring up anything other than the synth’s parameters under the ‘controls’ menu item in the mixer - I’ve tested using (of course) legal versions of Sylenth1 and Serum…
This is fixed (mostly) in the nightly builds. It is dependent on the plugin, but there was an actual bug introduced. I would suggest/request that you try the latest from http://nightly.ardour.org/