When you use the "Make Map" function in the Terrain Editor, it creates a bitmap named "map.bmp", which is located in the base directory of the terrain you are working with.
You have to rename the bitmap to whatever the terrain name is, then move it to the 'texsrc' folder.
For example, if the terrain is named landslide. You would rename the 'map.bmp' file to 'lanslide.bmp', then move than file to the 'texsrc' folder.
If the 'landslide.oba' file (this is the main terrain file) is located at C:\terrains\landslide, then the 'map.bmp' file is at C:\terrains\landslide\map.bmp. Rename and move it so it is located at C:\terrains\landslide\texsrc\landslide.bmp.
This all assumes you have created a terrain and populated it with tiles and have saved it.