Modding the game isn't directly supported under OSX, however it can be done.
First, edit MainMenu.lua, located in:
~/Library/Application Support/Steam/SteamApps/common/sid meier's civilization v/Assets/UI/FrontEnd
Comment the following line: (add a '--' to the start)
Controls.ModsButton:SetHide( true );
Copy a mod you have downloaded into the:
~/Documents/Aspyr/Sid Meier's Civilization 5/MODSdirectory.
You will need to unzip/extract them before they will work.
If it is a civ5mod file, simply rename it to .7z, and you can use StuffitExpander or any similar tool to extract the folder.
Start Civ5, and you should see a 'MOD' option on the menu.
Creating your own mods is easy enough if you know how. Armed with a decent text editor and the console you can create mods under OSX. Begin by reading Dale's explanation of modinfo files. Next, the 2K games Civilization 5 wiki is a good place to visit, in particular the Civ 5 XML reference guide. Finally, the civfanatics modders guide provides an excellent overview of creating a mod.
You can use the command line 'md5' program to generate the MD5 hashes required for the modinfo file. (And online lower-to-uppercase conversion)
You can download my simple casual gaming mod for Civilization 5 here. It replaces the standard 'quick' game. At the beginning of the game, you will find most things only take one turn to complete. Things will start to settle down quickly though in classical times.