The game should update automatically thus allowing you to play multiplayer.
You can speak to the live chat support, verify you own the game and brought the game and it is a disk based game, and they might be able to add it to your Origin account, and then you download it from there. They did this for me on my Battlefield games because I own them on disk and wanted on Origin due to disks not being safe to use I think because scratches are inevitible.
Anyway, Chat support are your best option, also
I swear the game auto updates, if not there is this: http://www.gamershell.com/download_72686.shtml
