I was able to enter the in the preferences > board mgr: http://arduino.esp8266.com/stable/packa ... index.json
in the tools>board mngr. window, with appropriate comma.
I opened the board magr, but could not find the ESP8266. When I went back to preferences it was gone. So I downloaded esp8266-2.4.1.zip file & unzipped it. It went into a file under ESP8266. How do I install it from there?