Using Steam version 4.0, latest build. 

Simply put, the tool is just not accepting whatever tilesets I attempt to import, including the ones I made specifically and even just random images as a test. Nothing shows up and I cannot even get started with this. 
I have followed all basic troubleshooting steps, including but not limited to relaunching Blender, reinstalling the plugin, and rebooting my computer. I seriously do not want to have to use an older Blender build just to use this.