Very much a PITA achievement as you can't just enter a gamertag to go to someone's locker, you have to find someone in a lobby.
From the main menu go to:
- Xbox Live
- Pick a lobby with people in it
- For each name in the lobby, pick them, then select "EA Locker"
If they have anything in their locker (roster, settings or created player) then download it and the achievement will be yours. If not (which is very likely the case) try someone else, eventually you'll get lucky and find someone who has something in their locker. Or alternatively, get a buddy and arrange a time to meet in the lobby so you both can do this.
Alternatively, according to Bill 13, you can "also go to the leaderboards and check out people's lockers from there if you can't find someone to work with. I just did it, found that number 6 or 7 from the top had a profile uploaded. I downloaded it and the achievement popped."