Can someone tell me how to create Impassable Terrain on the custom map

Can someone tell me the procedure to create Impassable Terrain on the custom map creator?

I would appreciate it - thanks.

It must be buggy tonight - because every impassable zone I am making right now is just dissapearing. I made some earlier and they stuck.

I started fresh from scratch and now I cannot make them at all. Weird. Any ideas?