5.6 was just released yesterday, so give the plugin author some time to get to it. Alternatively, you could try to fix the compilation errors yourself like I did. If you still have a copy of 5.5 installed, you can install the plugin and copy it to the plugin folder of your 5.6 project so you can edit it in your IDE.
I would give a more in depth guide for you but I’m not sure if I’m allowed to since I’m not the author.
Edit: Darker Nodes works perfectly fine (as far as I can tell) in 5.6 so you would just need to copy the installation from your 5.5 Unreal to your project’s plugin folder. The compilation errors only occur for Electronic Nodes.