For anyone who cant find it, go to the Play Station Store from the main menu (not in game) then search for 'B' select Battlefield then scroll down to AFTERMATH and click Add To Basket.
It then makes you jump through several hoops adding it to basket and go to checkout etc before finally the download starts.
And obviously its free for Premium players.
See you all in the new maps tomorrow guys.......after Ive sobered up hic!