HOWEVER, if this issue is actually for a Mod you've downloaded for the Civ 5 Live Tuner or other programs for the SDK, say so. If this doesn't work, then I'm sorry, but I'm clueless and don't know why it's being dumb.

Make sure you are subscribed to your Mods. Afterwards, redownload your mods via the in-game Mod menu. One thing: If you had files in your Mods folder from above and that doesn't work, try deleting them and repeating the process. Allow it to run it's course and then start up the game. If it's possible with the Mac Steam equivalent of Windows, try opening your Steam Game Library, right-click on Civ 5, and then go to properties (or whatever Mac has), then hit the Local Files tab, and finally click the 'Verify Integrity of Game Cache' button. If there isn't, then your mods are likely not downloading correctly. How does it work It parses your installed mods in the CiV userdir mods. Go in there and see if there's anything in there. The mod manager tries to enable mod support for Civilization 5 multiplayer games. In there, you should be able to locate a file folder fairly quick-like entitled Mods. If Mac has a 'My Documents' equivalent and/or a 'My Games' equivalent anywhere, try going there and locating a file folder for Sid Meier's Civilization V (with or possibly without the Sid Meier's part). If you have any experience with a Windows system, then this might be able to help you. I run on Windows and not Mac, so please, bare with me.