.maps do not become .pk3s.
.maps are turned into .bsps
the .bsp (and the rest of the relevant files) are thusly put into a .zip
this .zip is then renamed to .pk3 and pk3 is just a zip extention.
To simplify: find a archiving program that works for mac on a .zip format and just change the extention after you've put your COMPILED .bsp inside (retaining dir structure, ie. /maps/yourmap.bsp)