Steam should update the software automatically. Since the Steam Mac versions only appeared after the last patches produced by Aspyr, I imagine you got the latest version when you subscribed. I also doubt if there will be any more patches now.
Mods need to go into a Mods folder, and there are two of those in a normal installation. One is in the actual application's folder. The other is in a folder in your ~/Documents/<application name>/ folder, where '~/' means your Home folder, and <application name> depends on whether you are playing vaniila, Warlords, or BtS. Some graphics-only mods, like Blue Marble, can be installed in ~/Documents/<application name>/CustomAssets/
I don't have the Steam version of the software, but I *think* the application is installed in ~/Library/Application Support/Steam Content/common/ these days. It used to go in ~/Documents/Steam Content/common/.
Mods on the Mac are a hit and miss affair. There are a lot that don't work at all, or only work a bit, because the Mac version doesn't deal with custom DLLs which form the core of most mods. The rest may need fixes to make them compatible with the Mac version's XML parser or Python interpreter. Some of the ones that do work may prefer being installed in the application's Mods folder rather than the ~/Documents/ one. Check out the sticky mods thread in this forum for advice on which you can play.