The way that I usually do it is with a different FlxState - I use one for "Menu", the game itself, and the Game Over screen.
So make a new class that extends FlxState, call it maybe "MenuState" and then say:
super(resolution.x / zoom, resolution.y / zoom, MenuState, zoom);
Inside MenuState, on a button press or something, say:
FlxG.switchState(PlayState);