This is for map development. You need to pack your bsp and gunk in a zip renamed pk3.
Of course, if you thought about it for ten seconds you would have realized what I'm talking about. This is a mac only issue for those of us who are developing maps. It has nothing to do with further compressing a pk3.
On macs, directories have hidden .ds_store and other ._ files that store display and meta information for the directory. When you zip a directory with OSX's default archiver, those hidden files are added as well. Now, understand, that quake/trem will flip out at a pk3 with .ds_store files and will not be able to recognize your map.
So all this is, is a zip archiver that cleans out the .ds_store files. Makes life simpler.
Get it? Or do i need to draw you a diagram?
/set flame +scalding