Assuming we're both looking at the same editor, I think you just need to add the tileset to the level you're in. You can do that with a weapon script that calls addtiledef().
PHP Code:
//#CLIENTSIDE
function onCreated() {
addtiledef("lt_maintile.png", "levelprefix_", 0);
}
You can replace "levelprefix_" with "" if you want the tileset to be used for all levels.