This should work but, you’re right, my instructions aren’t very clear – I updated them soon after Gutenberg first came out but really need re-writing now.
If you head to the instructions and search for the FAQ named “CAN I USE THIS PLUGIN WITH THE NEW BLOCK EDITOR (AKA GUTENBERG) / WORDPRESS 5.0 AND ABOVE?”, that will explain it. Basically, there’s a block editor setting which will switch the custom fields back on, which should then appear at the bottom of the editor window.
Let me know if you have any problems with this.