Is there a folder that polds your steam folder on macs? If so just navigate to your steamapps folder, then your account name, then counter strike source and install skins like usual.
All the folders are the same in MAC as in PC, just in a different place. Its located in your home folder here: Library/application support/steam. When there its exactly like the steam folder in windows.
Depends on the file name. If the file is called Cstrike put it in your Counter Strike Source folder. if it is called Materials put it in your Cstrike folder.
If you have any problems as this didn't help add me on steam and I'll try to guide you through.