Zoom out until the view changes (other graphics, cloud cover etc.). Then look at the little buttons above the minimap. They have changed. Click on the leftmost (I think) button. A text menu will appear in the vicinity. In this menu, there should be an option to "Draw signs", "Name landmarks" or something like that (I don't recall the exact wording). Click it. Then click on the map tile you want to label.
If it's not the leftmost button, try the others - I'm not entirely sure which one it is.