You can't take a game same that another person creatred and use as your own, at least easily. You need to resign the save from another person to your psn account. Google the term "ps3 xploder". There are plenty of posts on this website pertaining to instructions on how to use it.

I have personally used many of the saves others have created to help finish games. Xploder is cheap, maybe 15 bucks. Money well spent.