I found that if I had a multi-row tileset it wouldn't go to the next row so I went investigating and I found out what the problem was:
Daniel messed up the math syntax
so replace:
tile_add(mappyTileImage, (mappyTileRemainder * mappyTileSize), ((mappyTileIndex - mappyTileRemainder) / (mappyTilesPerRow * mappyTileSize), mappyTileSize, mappyTileSize, (mappyX * mappyTileSize), (mappyY * mappyTileSize), mappyLayerDepth);
with
tile_add(mappyTileImage, (mappyTileRemainder * mappyTileSize), ((mappyTileIndex - mappyTileRemainder)) / (mappyTilesPerRow) * mappyTileSize, mappyTileSize, mappyTileSize, (mappyX * mappyTileSize), (mappyY * mappyTileSize), mappyLayerDepth);
and that should remove that nasty little error.