How can I reset my MainMenu.lua file?

Right click the games list entry of the game in Steam, pick "Properties", go to the "Local Files" tab and let it verify the files. Once this is done it should note that it found modified files and downloaded them.

After that everything should be back just fine.

If the file is part of some third party mod or some local copy overriding something else (no modding experience for Civ 5 here), then you'll have to delete or restore it from where you've got the original copy.

If you're using the non-Steam version, you may have to back up your save games, uninstall Civilization, and re-install, unless your friend made a backup or you can get a copy of the original unmodified file for your version of the game.

If you're going the route of backing up your games and re-insalling, the Mac app store version keeps its saves in ~/Library/Containers/com.aspyr.civ5campaign/Data/Library/Application Support/Civilization V Campaign Edition/Saves. You may have to make your library folder visible before you can access it.

Mac Steam Civilization 5 saves are easier to find, in ~/Documents/Aspyr/Sid Meier's Civilization 5/Saves (which I mention mostly in case they decide to standardize the location between the two versions).

If you have a backup or can get the original file from a friend, and want to try restoring it manually, right-click your Civilization app, and choose "show contents". Inside the app, you'll find MainMenu.lua in Contents/Home/assets/UI/FrontEnd.