It's a bit tricky how it is now but they will make it easier soon, a Sync to Host button should be added and you will just left click the track in the BB Plugin to select (opt+left click will deselect all) and the track color will change.
To play the selected tracks in the BB Plugin without the DAW playing you just hit the BB Plugin's Play button.
In the video I double click the drag icon to enable synced playback.
The BiabVST track is sending channel 8 midi to the Piano VST track.

Watch Video
https://www.dropbox.com/s/i8riwunixipytnn/Mac-BB-Plugin-Send%20Midi.mov?dl=0

Pic below is how it should work when fixed: