I noticed that the Civ5 installation includes a Resource folder, and that contains a number of .fpk files. Each of these files contains multiple files, and they seem to perform a similar function to the AssetsX.fpk files used in Civ4, but the format is not identical. So I have adapted my Macapaka utility to extract and create files in this format.
Download Macapaka version 2.0.0 if you want to explore these files.
I understand that the Nexus component of the Windows SDK is able to process some of these files, and a fan-created Windows utility does the others. Now you have one for the Mac
I have not tested it to destruction, and some of the .dds files it extracts don't open in GraphicConverter, but I haven't investigated that to determine whether the files are corrupt or GC is just limited. Please let me know if you find any problems with it.
Download Macapaka version 2.0.0 if you want to explore these files.
I understand that the Nexus component of the Windows SDK is able to process some of these files, and a fan-created Windows utility does the others. Now you have one for the Mac

I have not tested it to destruction, and some of the .dds files it extracts don't open in GraphicConverter, but I haven't investigated that to determine whether the files are corrupt or GC is just limited. Please let me know if you find any problems with it.