What these instructions look like depends on whether NoahAndrews's setup scripts get merged
If you're using homebrew, you can use the following commands:
brew tap osx-cross/avr
brew install avr-libc
brew install dfu-programmer
Otherwise, these instructions will work:
Xcode->Preferences->Downloads.If you have any problems building the firmware, you can try using a tool called Vagrant. It will set up a virtual computer with a known configuration that's ready-to-go for firmware building. OLKB does NOT host the files for this virtual computer. Details on how to set up Vagrant are in the VAGRANT_GUIDE file.