If you are using the Arduino IDE 1.8.x and you want to import a board or a platform then you need to use the Boards Manager.
Follow these simple steps:
- Launch the Arduino IDE 1.8.x.
- Select Boards Manager... from Tools>Boards menu.
- Search your board or platform.
- You can scroll the drop down menu or write the name in the top field of the page.
- Check the version and click on Install.
- Wait that the installation is completed
- Now check your board in Tools>Boards menu.
If the new board doesn't appear, then restart the IDE.