Adding Attiny to Arduino IDE - attiny\variants\platform.txt file missing
Open the "Preferences" dialog in the Arduino IDE.
Click on the symbol next to "Additional Board Manager URLs"


Paste the following into the box (possibly on a new line under any existing URLs), then click OK.

https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json




Note that you can click on the circled link for a list of boards. This will open in your web browser. You may have to switch to your browser to notice it.





Click OK to close the Preferences dialog box.
In the Tools menu / Board select the Boards Manager:


Scroll down until you see the Attiny entry (or the one for the board you want), click on it to select it, and then click "Install".


Once done, close the Board Manager dialog box.
You should now be able to select Attiny from the Boards menu:


Now in the Tools menu you can select which Attiny you want to use:


Notice you can also select the appropriate clock speed in an adjacent menu item.
To program the Attiny you will need a suitable ISP (In System Programmer) such as the AVRISP programmer, or the USBtinyISP programmer.