I am having the same problem. I was unable to select a category on the promotions page for some reason. Link to screenshot http://www.antennaengineering.co.uk/nocats.png so I went a different route and created posts where I could choose a category but still no joy. Link to screenshots http://www.antennaengineering.co.uk/shortcodecat.png, http://www.antennaengineering.co.uk/postcategoryselected.png.
It shows all posts in the first slider I created then just the selected ones in the latest slider it created.