For third party hardware packages, it is first necessary to add the URL of their Boards Manager JSON file in: File > Preferences > Additional Boards Manager URLs. The URLs point to JSON index files that Arduino IDE uses to build the list of available installed boards.
If you have multiple URLs in that field, you must separate them with commas. New boards will automatically be picked up by the Board Manager each time Arduino IDE is opened.
To find the most up to date list of URLs you can add, you can visit the list of third party board URLs on the Arduino IDE wiki.