The IWP documentation says "Arrange fields formatted as a checkbox set or radio button set with the value lists displayed horizontally for best results."

How do I do that?

At first I thought in my layout I would make it horizontal by restricting the box to be wider than tall, but that doesn't help. So there must be a more tricky way that I don't know how to do.

I figured out the problem, I didn't have value lists accessible by my web user so the radio button set wasn't appearing. I enabled the value lists and then things worked as expected.