If you are playing One 4 Nine on a Mac and the skybox is being displayed as a large black/purple checkerboard, follow the below instructions.

Download this file, which contains the skybox files missing from the Mac version of Left 4 Dead 2.

The default place for Steam to install to on a Mac is in the *specific user's* Application Support folder (not the all-users one).

Copy the l4d2_one4nine.vpk file to the following location:
~User/Library/Application Support/Steam/steamapps/common/left 4 dead 2/left4dead2/addons

Put the sky texture files from the sky_day01_09_hdrbk.rar file to the following location:
~User/Library/Application Support/Steam/steamapps/common/left 4 dead 2/left4dead2/materials/skybox/

If there is no "skybox" folder there, created one, name it "skybox" and copy the sky texture files there.

Many thanks to Tempe for testing this fix.