There are 4 maps to be played as part of the Outer Rim DLC, they are only playable from the separate 'Outer Rim' playlist which is bottom left of the main multiplayer menu in the 'Expansion Pack Playlists'. You only need to play on all 4 for the achievement and not win on them, you can actually just load up into each map and have that count for the achievement but you cannot choose which map you play on.
The maps you need to play on are:
• Jabba’s Palace
• Palace Garage
• SoroSuub Refinery
• SoroSuub Pipelines