I had issues like this for a long time, figured it out by going to steam, going to properties of the game, browse game files, copy everything to your host file. Follow all of the steps of adding files into your bin / root folder in the original post. I have found that I need to either copy the folder into themselves or pay extra attention to ctrl+drag and then 'update if newer/merge' options when copying the files to prevent directories being replaced with incomplete ones (one of the nightmares of the mac file copy routine). Whole process usually takes me about 15mins and I must rebuild the server files after every update manually
On another note has anyone got any idea where to begin with compiling source mod extensions/addons to mac OS X. I'm really keen to get Left4Downtown working on my Mac for LAN parties, but no one seems to have attempted and I can find any guides for building the Mac Binaries. Any help would be greatly appreciated - not the typical 'compile for mac' answer I've seen, as no one seems to have done that, and I have no coding experience. A guide for compiling these for Mac would be perfect if someone knows how or has a link.