The mac version is self-contained. It's a package (like nearly all OSX applications) and is simply opened from the right-click menu. Macs come with all that is needed to mod CC, and open archives, and all that jazz. That said, I recommend 7zX as a better archive-opening program.

STEP 1:
Right-Click or control-click the icon, and select "show package contents."

STEP 2:
Open the "Contents" folder.

STEP 3:
Open "Resources." You may also Right-Click this folder and choose to create an alias, with eliminates the previous steps for the future. Put the alias somewhere else, named "Cortex Resources" or something.

STEP 4:
Now that you have resources open, you simply place extracted mods in here.
Mods should be a folder with the file extension ".rte", like this: "base.rte"

DO NOT screw with the files that are there in the original install of CC unless you actually know what you are doing.
However, I encourage you to open and read the ".ini" files, these are what define the objects used in the game.
These are simply text, you may open them with the default editor, or download and use the superior TextWrangler.
For example, you would edit the "Skirmish Activities.ini" in "Missions.rte" to modify the variable for the starting funds for a given team in the skirmish.