Take an "id1" folder from an existing installation of
Quake and place it next to vkQuake.app.
To run different mods, you will need to drill into the vkQuake.app/Contents/MacOS directory in Terminal and run the vkquake executable with command line options. In the future we hope to add a GUI for this.
For more information on getting the game's data onto your Mac check our
FAQKNOWN ISSUE: There have been reports that double-clicking "vkQuake.app" does not run the game. I am looking into why this happens but in the meantime if you run into this you can drill into the package contents and run the executable directly. For more information see boggydigital's guide
here and Andrew Tsai's video
here