Help with map editor

I'm pretty new to this side of the game, and now I'm hoping to create a map of my own. I've started with a copy of one of the game's base maps, and managed to get rid of the assets I don't yet need, but when I go to create a terrain block all it does is remove the collisions from the existing one. Nothing new shows up. What am I supposed to do?

Edit: I have a solution to this first question, scroll down to find the new problem

 

I am also pretty new to this part of the game. I cannot figure out how to get object coordinates.

 

Update: I'm ok with terrain manipulation now, but how do I import/create assets (mainly road decals atm)?