Just wanted to note that .civ5mod files are actually .7z compressed archives. I don't think OS X's built-in decompression tool will handle it, but there are some good options, this way to can bypass the step of having to expand mods in a Windows version of Civilisation V.
Just install a compression tool that handles .7z for Mac (I use BetterZip), then change the file extension of the mod to .7z, and double-click.
The only difference in this method versus Civilisation V doing it is that you'll be left with underscores in the folder name instead of spaces, but you can remove these manually if desired, though I don't think it matters.