Basically, used this methods to share my map pack code, with most of my friends online.
One person has to buy the code or already have it installed.
Then to install it on your system just login to their PSN account, go the Playstation Network Downloads and just download the map pack onto your PS3.
Then login into your account and play to your hearts content.
Hope this helps i also used this when i bought a new PS3 and needed to re install the maps.