Use Taxonomy from plugin in Post Slider

Hey guys, I’m looking to achieve a “post slider” of upcoming events that are created in The Events Calendar, however I can’t use the taxonomy selector in the Post Slider widget to grab a specific category within The Events Calendar. The look I’m trying to achieve is like the one here: http://www.humphreysbackstagelive.com/

I’m using a Child theme, and I’ve added custom taxonomies before but never to be used with a plugin, so I’m just wondering if you can steer me in the right direction? Thanks in advance!!