You use the "KStars Settings" menu in "Settings" above the SkyMap, and go to the "Terrain" section.
Select the file you want (defaults to the default directory for terrain) and set the azimuth correction to match the image and the sky (doesn't matter for Mars, but matters for your own surroundings) and check "Show Terrain" and click on "Apply".

If you have your own file, it's good, but not required, to put it in the same directory as the terrain settings file-chooser defaults to.
 

Hi Hy - this Terrain is fab, any news on the altitude adjustment for it ? I'm still battling to get the image lined up with my horizon and it's taking a long time with the manual back and forth of image editing  

@JiberJaber, @JerryBlack: I recently submitted MR 385: invent.kde.org/education/kstars/-/merge_requests/385 which allows for vertical adjustment of the terrain image, as you both requested.  It's in the beta software now. If you use that, you should see the following in the Terrain options:

 

The bottom line in the "Source Options" section, "Terrain Image Altitude Correction Degrees" allows you to enter the altitude correction.

If you enter a positive number and hit apply, it will raise your terrain image by that many degrees in altitude (yes, I know, thus causing a little, hopefully harmless, transparent hole below us, at -90-degrees altitude).  A negative number similarly lowers the terrain image. This setting has no effect on any artificial horizon lines you may have entered. It is purely visual.

Hope this helps,
Hy

Thanks Hy - I'll have to wait till it makes it in to StellarMate but looks like it will allow me to align the visual terrain with the horizon lines which is great!

This is while using the terrain feature in the equirectangular projection. There is obviously something not looking right.

Thanks for reporting this. I found the issue and sent Jasem a patch for review.
Hy