Adding a layer to a map

Stupid question I'm sure, but i simply can't figure it out: How do you add a layer to an imported map?

I created a map in Fractal Mapper showing an attacked wagon and then imported it in d20pro. The wagon on the map is roofless because I wanted to add the layer in D20pro, then paste the wagon roof on that layer as to be able to show/hide it during the game when the player search the scene. But for the life of me, I simply can't figure out where in the Draw Tools I can add a layer when it's an existing map!

Help! :?

 

Finally found it some minutes after posting. Ctrl-Click