I selected the region I wanted to copy and paste. I checked Mirror-X. It didn't mirror the region on the X axis when I pasted it.
Help anyone?

Mirror only works on Isometric terrain placement, not square or copying.

What you could do, if its a large region of the map, is somehow use tools like SCPM to get one side of the map into a picture file, then you can load that into another prog that will make the mirror. Ofc you would have to go into mircosoft paint or what havce you, and flip it horazontally.

thats what i would do. unless its a really small bit, then you might as well just do it manually