I hope what i am going to say is right.
Well...Right click on the map (in the small, white box in the left. All of the other already made maps are there to.) and then select "Create Area". Now, where the "..." is under the word "Boundries" click it and se;ect the ammount of land you want in that area (in this case, the ice-forest section of the map) and then assign enemies to it via the box in the right hand corner.
Just don't make encounters happen in the entire map, as well (while you st up the main map)