What he's trying to say in english is...
If you have a terrain called "myterr" there is a folder for it. Create a folder inside of that one called texsrc. So your folders would look like this...
Program Files
....HTC
......ah2editor
.........ah2terr
.............myterr
.................texsrc <--- You create this folder
Save the bmp file he posted into that folder.
Now, the terrain editor has a cache folder. Like this...
Program Files
....HTC
......ah2editor
..........cache
Delete all of the files in the cache folder. Then start the editor and the tile you just saved will show in the editor.