(This answer is based on my experience with the trial of the pack. It probably isn't any different than the official release.)
The maps found in the Mediterranean Traveler Map Pack will not be found in the normal game type categories. There is a separate category in which you can find when trying to play a Custom Match. Here, all the "extended maps" (current and possibly future) will be in the playlist for all the game types.
In the Custom Match menu, change the Simulation Category from the default categories ("Simple Deathmatch", "Free For All", "Team Objectives", and "Private") to one of the two new categories. There's one for "Extended Maps" and "Private Game using Extended Maps" (I don't remember the exact wording, I didn't purchase the DLC yet). Then you'll be able to create and join these games using the new maps. It will only play these maps exclusively and not include the regular maps in the rotation. (They do this probably because they don't want to mix people that don't have the DLC with these players so there's no confusion about what maps get played).