iMovie 10.1 for El Capitan fixes Theater & iCloud BUT...
I along with all the other early adopters of El Capitan have been struggling with the inability to use Theater and iCloud with iMovie for the last few weeks.
The new iMovie 10.1 that was released today (10/13/15) fixes all of those issues.
But, BEWARE. The upgrade to this new latest version of iMovie messes with the Event and Year structure again. It took all my year based events and lumped them back into one big Event under the name of my library.
Ugh.
Good luck everyone!
MacBook Pro with Retina display, OS X Yosemite (10.10), Mid 2012 2.7 GHz i7 16GB 750GB