5. Expand/Extract the Mod
Mac Civ5 cannot handle a compressed mod file. It must be extracted into a folder of plain, uncompressed files before you install it.
Steam Workshop mods will download as compressed .civ5mod files. These files are compressed using '7zip' format (.7z). Other mods may be available in a number of formats - .zip, .7z, .rar. These are all single files that contain the separate files that make up the mod.
If the mod downloads as a .zip file then you can use your usual unzip utility. Safari may well unzip it automatically, depending on your settings.
If the mod downloads as a .civ5mod or .7z or .rar file then you can use Keka or Ez7z to extract it.
I have now found Keka, which seems to work well. It is still available at MacUpdate, and downloads as a disk image file. It is probably a better alternative to Ez7z, but I suggest that you set its General screen to "Always extract", otherwise, if you drop a .civ5mod file onto it, it will not recognise it as a compressed file, and it will compress it again!