Double click the landscape sprite to go to the image editor.
Select the polygon collision tool (under hotspot tool).
You should see a bounding box with 4 vertices.
Right click the image and a popup menu will appear.
Select guess polygon shape (it won't match exactly but it's a start).
Right click the vertices and select "add point" from the popup menu.
Move the points to fit the terrain.
From the looks of things, the collision polygon isn't editable for TiledBackground.
Here's a little capx for you.