Open the .mis with notepad, and look for this.
new TerrainBlock(Terrain) {
rotation = "1 0 0 0";
scale = "1 1 1";
terrainFile = "./Map_Roco_Island.ter";
bumpTexture = ".";
squareSize = "8";
bumpScale = "7";
bumpOffset = "0.01";
zeroBumpScale = "6";
RepeatTerrain = "1";
locked = "true";
};
change
terrainFile = "./Map_Roco_Island.ter";
to
terrainFile = "./Roco_Island.ter";