This gave a hint to a problem. First, I confused in my description "left" and "right" click, because I have set my mouse to left handed. More importantly, I found that with what is commonly known as "left click", it indeed DOES add a bookmark, but the problem is that the "left margin" is not, as I thought, the area with the line numbers, but a very tiny (small) area between the line number and the edit pane. Since it also is coloured the same as the line number area, it is virtually impossible to see. I just found it by accident.