Unfortunately, some PS3 premium owners seem to be experiencing a bug:
NOTE FOR PS3 PREMIUM MEMBERS: Currently, your copy of Armored Kill is available on PlayStation Store but not in the in-game store. We are investigating this matter and should have it up in the in-game store eventually. In the meantime, download your expansion pack on PlayStation Store.
For more info on Battlefield 3, please visit here: http://www.battlefield.com/battlefield3