I was able to find a solution per the EA website:
QUESTION
How can I place custom maps in Command & Conquer 3 Tiberium Wars for the Mac?
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Answer
Any custom maps will need to be placed in the Command & Conquer 3 Tiberium Wars folder on your Mac. When installing a map, you must ensure that the folder that you create is the exact name of the map itself, and all map files placed are within that folder, or else the game will not recognize the new maps. Once the map folder is created, you will be able to access the new map in the Map Selection screen for Skirmish or Online.
All custom map folders must be placed in the following folder:
* (Your Username)\Library\Preferences\Command and Conquer 3 Preferences\p_drive\Application Data\Command & Conquer 3 Tiberium Wars\Maps
Hope this helps you guys!