Mods are at:
/Users/<your_user_name>/Documents/Aspyr/Sid Meier's Civilization 5/MODS
OK, found it
DLC is at:
/Users/<your_user_name>/Library/Application Support/Steam/SteamApps/common/Sid Meier's Civilization V/Civilization V.app/Contents/Home/assets/DLC
To get to the DLC you may have to find the Library folder by holding down the Alt/Option key while selecting Finder->Go. Library will appear in the menu list. Select it to open /Users/<your_user_name>/Library/
I can find the DLC folder...but I cannot find Library. When I hold "alt/option" and click finder then type "DLC" the folder comes up....I click it...but do NOT see a library folder
You then have to navigate to the Civilization V.app item at Application Support/Steam/SteamApps/common/Sid Meier's Civilization V/Civilization V.app. Right click it, and select Show Package Contents to display the Contents folder.