You must trade it via Local Wireless and you can only trade Darkrai, Riolu and the Manaphy Egg one time. After that you can never trade them ever again.
To transfer a Darkrai, Riolu or a Manaphy Egg to your Pearl, you must go to your Mystery Gift via Pearl and choose the option Recieve Gift from a Friend.
Did you already trade your Darkrai to your Pearl?
Did you use an AR while playing your Almia at all?