I downloaded my copy of Civ 5 from Steam (I have a Mac) and I do not seem to have the scenarios or mod options that are supposed to be there in the main menu. Is this just because it's the Mac version or is there some other reason? Thanks!
It's because you have a Mac version, on which the mod browser won't work (link). DLC is not currently available on Mac versions, but Aspyr's working on it.
Note that if you have bootcamp on your Mac, you can install the Windows version there (on which mods and DLC work) for no additional charge.
Official scenarios are now supported on the Mac version of Civilization V.
Mods are still not supported, though some of them can still be ran with some hacking; see How can I enable Civ 5 mods on Mac OS X?