> I tried it again, and that time nothing happened, that is, I selected two
> points not connected by a line, chose join, and no line appeared.
That's not what join is supposed to do. From the docu on 'join':
It looks for any paths with endpoints as the endpoints of other paths
and then join those two paths. Also if the endpoint of a path is the
same as the start point it will make that path into a loop.
So the two points have to be in the same position. What you are trying
to do can be done with one of the point adding tools. Just select one of
the points (pointer tool) you want to join and click with one the second
point (point adding tool).