This repo contains the custom package_*_index.json files that can be used to add new third party boards to the Arduino v1.6.4+ IDE.

To add MySensors to Boards Manager list, go to IDE preferences, and at the bottom of the dialog add the following URL into "Additional boards managers URLs:" field and hit OK:

https://raw.githubusercontent.com/mysensors/ArduinoBoards/master/package_mysensors.org_index.json

For the Sensebender Gateway, you'll need to install the Arduino SAMD board as well. Otherwise, you'll get this error:

The current selected board needs the core 'arduino:arduino' that is not installed. 

https://github.com/arduino/Arduino/wiki/Unofficial-list-of-3rd-party-boards-support-urls