This capability is supported in LR4, but not LR5:
Adobe reworked the LR5 Slideshow module and new pictures are no longer added automatically. You can install LR4 in addition to LR5 and use them both.
Thank you for the answer. Although I can't understant that this is true - less performance in a new version...
Try to turn back to LR4 instead.
Marc de Fotograaf.
I believe the issue that was addressed with LR5 Slideshow is image quality. The images must be properly rendered before they can be displayed as a slideshow. They added this "improvement," but dropped ability to add new pictures real-time.
You can add a "feature request here to restore this capability that was in LR4:
I am also looking for this option in Lightroom 5.7 Also want to startup my photobooth in the same way as Mark.
Is there no other option to do this ??? capture tethered pictures in Lightroom and show them in real time.
other software may be???