On macOS, install the Arduino IDE using [homebrew](http://brew.sh/) [cask](https://caskroom.github.io/) with `brew cask install arduino` or download the application from [the official website](https://www.arduino.cc/en/Main/Software) and move it to your `/Applications` folder.
On Linux, your distribution's package manager probably includes the Arduino IDE, but it may be an out of date version. If your distribution installs a version of Arduino before 1.6.3 or so, you'll need to install the Arduino IDE following the instructions [on the wiki](https://github.com/keyboardio/Kaleidoscope/wiki/Arduino-Setup-Linux), installing the Arduino package somewhere that 'arduino' and 'arduino-builder' end up in your $PATH