So back to doublecad.com to ask for the number. It's not free.
Downloaded DoubleCAD XT. Last step requires customer information, which asks for serial #. The user agreement says it is provided with the software, but I can't find it.
Light bulbs you shouldn't buy
There are plenty of dimmable LED light bulbs, but make sure you don't buy the ones that flicker when you dial them down.