Thanks for taking a look!
Can you place it in the transparent background using GIMP or Photoshop? That's much better if you can because you can resize it or edit it if necessary, which you can't do that well if it's actually part of the render.
I don't mean to sound frustrated, but I wrote this tutorial 5 years ago and can barely remember what I ate for breakfast this morning. If I didn't cover it here, I probably don't know the answer. Again, I would suggest that you post in the DAZ Commons and see if someone who works with canvases more often than I do might be able to help.
It still works for me. I put a plane and a sphere in a scene, attached each one to a node, chose a Beauty canvas type and rendered to a new window with the Spot Renderer. The problem likely has something to do with what objects you have attached to which nodes. Make sure to check which node is selected as well (under the Alpha checkbox).