{
	"text": "改行A\\n改行B"
}

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" xmlns:s="library://ns.adobe.com/flex/spark"
			   xmlns:mx="library://ns.adobe.com/flex/mx">
	<fx:Script>
		<![CDATA[
			protected function btnLoad_clickHandler( event:MouseEvent ):void {
				var loader:URLLoader = new URLLoader();
				loader.addEventListener( Event.COMPLETE, loadComplete );
				loader.load( new URLRequest( "./sample.json" ) );
			}

			private function loadComplete( event:Event ):void {
				var loader:URLLoader = event.target as URLLoader;
				var obj:Object = JSON.parse( loader.data );
				var value:String = obj.text as String;
				var reg:RegExp = /\\n/g;
				lblText.text = value.replace( reg, "\n" );
			}
		]]>
	</fx:Script>

	<fx:Declarations>
		<!-- 非ビジュアルエレメント (サービス、値オブジェクトなど) をここに配置 -->
	</fx:Declarations>
	<s:VGroup>
		<s:Button id="btnLoad" click="btnLoad_clickHandler(event)"/>
		<s:Label id="lblText"/>
	</s:VGroup>
</s:Application>

RegExpを使って正規表現で\nを\nに置換することでLabel内で改行して表示される