Pixel of Life

This can be done with SteamCMD, Steam's command line interface. Download/install it, then run it and do these commands:
login "username" (Without quotes, replace "username" with your Steam username. It'll then ask for your password, enter it and it'll log you onto Steam) force_install_dir "path" (Again, without quotes - replace "path" with the path to a folder you want SteamCMD to download the game to) app_update 220200 @sSteamCmdForcePlatformType macos validate (220200 is KSP's App ID, the next command forces it to download the OS X version of KSP, and validate makes sure all the files downloaded correctly and are not corrupted) You should now have the OS X version of KSP in the folder you specified in the force_install_dir command. You can also substitute the "macos" in the ForcePlatformType command with "windows" or "linux" to download windows and linux builds, respectively. This works on all three platforms and for any Steam application, all you need is the App ID of the application you want to download. They can be found on SteamDB.