Could you post a screenshot of the problem?
Also does that happen with all images?
It's a known problem with 6.4. Go to Develop or Slide Show and run the slide show and it will work. I found that after doing that, the quick slideshow in Library started working, at least for that run of LR.
You need to select a Template first in Slideshow module.
Switch over to slideshow module and select any template first
Then select Impromptu Slideshow or CTRL+Return.
Thanks guys - did not test closing and opening LR to see if it stuck but this works OK for me.
Choosing a temple.....this worked fine for me!