Michael Baxley just pointed out that:
Warn readers to NOT update Lightroom 6.4 because it kills Photomatix Pro. The support page says engineers are working on a fix.
I just got my update notice today (January 27, 2015).
Update Jan, 29, 2016: I just verified Michael's report above. I upgraded to Lightroom cc 2015.4 and tested Photomatix Pro before and after. Worked fine before the update but after the update after the first screen, Photomatix Pro does not run. I'm using OS X El Capitan 10.11.3. Don't know if this is a Mac only issue. Problem seems to be Lightroom is just not starting the Photomatix Pro plugin.
I also tested LR/Enfuse and it still runs ok. If you use Photomatix Pro you should not update to LR 6.4/c 2015.4!