I've recently updated Lightroom CC to 2015.4 (MacOSX 10.11.3). When I try to run an Impromptu Slideshow I get an error message that tells me that video support is disabled. Is there a fix for this? It is a new problem.
Go to the slideshow module and click on any template in the left-hand panel. Then go back to Library and it should work.
That solved my problem. Thanks.