Most parallax units are "hidden" from the editor, because they are very annoying to have floating around the editor when you work with them.
See the selection bounding box is very big and also "invisible" since the actually graphics could be in a completely different place, this makes it more or less impossible not to select or move clouds by mistake when you work in levels where they are.

To enable these units, you need to "mod" the selection back. All clouds are placed inside the "/terrain" folder, which in turn are inside "/doodads". So, what you will want to do is, copy let's say cloud_egypt.unit and place it inside your own scenario folder and then at the same place, so like myscenario/doodads/terrain/cloud_egypt.unit. Then, you need to open this file and replace <unit layer="100"> into <unit slot="doodad" layer="100">. Save that file, restart the editor and open your map, you should now be able to place those clouds.

Thanks, this clears up everything!