So I am also on a mac, and have struggled with modding OpenSpades for a long time. And although this is an old thread, im commenting in case anyone else wonders how to do this.
So in contrary to what others have said, I found that the resources did have to be compressed in the .pak format, and could not be unzipped edited and recompressed as the game didn't seem to accept them. For those who don't know, the resources are found in OpenSpades.app You right click and click on show package contents, then Contents, Resources and you'll find a bunch of files in the .pak format.
So first of all you'll want to go ahead and download BetterZip (you can find it on their website, it'll be your first google result). Once you have that downloaded, you'll want to open it, and open the BaseSkin.pak (or whatever .pak file you wish to edit). It will open up a new window, which youll be able to navigate like a finder window. You'll want to make any changes by simply dragging the items into the required folders from Finder. Once you're done modifying the game just as any other users (mostly on PC) would (you can find lots of tutorials for that online) you'll want to click Save up the top. It wont offer a .pak save option, but thats fine. Simply save it as a .zip file back in the resources directory. Once thats done navigate to the directory in Finder, delete (or backup, i.e move out of the folder) the old .pak file, and move in (if you didnt save it there) the .zip . Once its in place you can simply rename it with the .pak file extension, and you're done
After thats done, go ahead and open the game, verify it works, and enjoy your mods
Hope this helped someone,
IEMatrixGuy Out