I dont think its supposed to be saved in Texures, Terrains in the map file is where i save mine. Try saving it there and to open it go onto map maker, the World tab, and type in the name of your map in the defaultTerrain.png e.g. if your map name was ThePlains or something type it exactly like u typed it in the Map Name thing, so ThePlainsTerrain.png. Hope that helps i can give a better description if u want.