So it turns out that the problem is my operating system- guess I have to upgrade before the new Sketchup will work. Hope this old machine will take one more upgrade. But thanks for your help on this.
If I may, there is a new question. I've installed the Bezier Spline plug in, because in my woodworking I use a lot of curves. But now I'm like the cat that's caught its first bird- I've got no idea what to do with it. Are you aware of any places/sites that talk about which spline choices are best for which tasks, and actually how to use these things?