@cavilesn A mod is a modification to the game, a file with extra instructions that the game loads along with its program files. Mods can add extra functions, or unblock functions (like allowing you to edit an outfit that normally can't be changed), or prevent things you don't want happening; there are almost endless possibilities. Some mods are poorly made and shouldn't be used, but NRaas mods are all high quality and have been tested by tens if not hundreds of thousands of player over the years. I've used MasterController (and several others) for almost a decade with no ill effects.
The way to install mods is the same on a Mac as it is on a Windows PC. Here are useful instructions:
http://modthesims.info/wiki.php?title=Game_Help:Installing_Sims_3_Package_Files/Setup_and_Files
I can't directly link any specific mod on this site, but if you google NRaas MasterController, you'll find it easily enough. I believe you only need the base mod to change a career outfit, but I also have the Cheats add-on installed.
If you'd like help setting up mods, the main NRaas discussion forum (called Chatterbox) is a great place to ask. But the instructions I linked above should get you started.
