Also, the instructions were unclear, so I was unable to distinguish how to get the popup on page load. A waste of my time.
I am sorry to have wasted your time...
However I could have helped you in a blink if you'd asked...
As your lightbox uses jQuery there would be a jQuery conflict since Simple PopUp uses FancyBox. There is a setting to turn off jQuery loading for Simple PopUp which would have removed the conflict.
This is also described in my FAQ and in the documentation.
Simple PopUp is a PlugIn and thus built to reside in Articles. You can use the extension "Articles Anywhere" to include it into a module.