To use this as a background image simply create a new instance of TmxEntity.
public function createMap(mapData:String)
{
// create the map, set the assets in your nmml file to bytes
var e = new TmxEntity("maps/mylevel.tmx");
// load layers named bottom, main, top with the appropriate tileset
e.loadGraphic("gfx/tileset.png", ["bottom", "main", "top"]);
// loads a grid layer named collision and sets the entity type to walls
e.loadMask("collision", "walls");
add(e);
}