Thank you very much for the help. It worked like a charm.
under ‘windows’ select ‘layers’
The Layers window was already visible by default.
optional: Load the ‘Openstreetmap (Carto) Standard’ or similar from the imagery tab - might d to find it under imagery preferences. Also use the blue arrow in the layers box to put it as the bottom layer.
To display a map in the background, open the Imagery menu, and select a map (eg. “OpenStreetMap (Mapnik)”).
To move the map, press the right mouse button instead of the (more common) left mouse button, and pull the map.
Open your waypoints gpx layer, it can then be seen in the layers window as ‘Markers from…’, right-click it and select ‘Convert to data layer’ so that you can select them them with lasso select tool at left of screen (or the normal rectangle select button), zoom into your gpx track.
When right-clicking on a GPX layer > Convert to data layer, JOSM displays a dialog: “Warning: Upload of processed GPS data as map data is considered harmful.”. Ignore it, and just click OK.
At this point, the markers will turn from pink crosses to clear blue dots, making them invisible when zooming in with the map displayed in the background.
Activate the ‘Markers from…’ layer by right-click and selecting activate.
Actually, at this point, the layer is now called “Converted from: Markers from waypoints.file.gpx”
“Convert to GPX layer” displays the markers in pink with their names displayed.
Here are screenshots I took while going through the motion:

The GPX works fine:

I have a couple of questions:
-
What does Activate do? How do I unactivate (no item in pop-up menu)?
-
Is it possible to make markers more visible, by changing their shape (cross), size (small), and color (pink)?