The following code that I found in an old thread displays a Mochi ad and then switches the scene when finished. Note that "2" is a scene ID and you'll need to change that to your next scene's ID (check the console to see it).
MochiAd.showInterLevelAd( {clip:FlxG._game, id:GameState.mochiID, res:"640x480", ad_started:function():void{Mouse.show();}, ad_finished:function():void{switchScene(2, createFadeOut(((1000*0.25))), createFadeIn(((1000*0.25)))); Mouse.hide(); }});