Adding

A FlxObject will not appear on the screen, until it has been added to the current FlxState (or a child of the current state).

The order a FlxObject is added determines their order within that object’s members. Typically, each FlxObject is drawn, and updated, depending on this order, with all the children of an object being updated/drawn when their parent is.

// add object to the current state
add(object);

Demonstration

Source

View Source on GitHub 

package;

import flixel.FlxG;
import flixel.FlxSprite;
import flixel.FlxState;

class PlayState extends FlxState
{
	override function create()
	{
		super.create();
		
		var box = new FlxSprite("assets/bigbox.png");
		box.screenCenter();
		
		var sprite = new FlxSprite("assets/sprite.png");
		sprite.screenCenter();
		
		add(box);
		add(sprite);
	}
}