To add a legend in CadnaA, you must follow the following steps:
1) Select the symbol

and draw a rectangle on the canvas.
2) Select editing mode and double-click on the edge of the box drawn in point 1. The following pop-up should appear:
3) In the "Symbol" field, select the Legend option that is most appropriate:
If "Legend: Mesh" is selected, a level legend according to the selected Mesh configuration: