To access your purchased maps please do the following:
1. From the main menu, go to the multiplayer mode.
2. Access the Playstation Network option.
3. Select the Create a Game option.
4. Hit the Triangle button.
You should now see your purchased maps: Little Bighorn, Fort Smith, Elmira Prison Camp and Vulture Mines.
5. You will need to add them to your list of default maps to access them.