Additional boards manager option is missing from Arduino IDE
You need to download and install a recent version of the Arduino IDE from:

http://www.arduino.cc/en/Main/Software

When you install the Arduino IDE using apt-get you get a very outdated and non-standard version, which does not have the Boards Manager feature. The reason for this is that the license documentation of the Arduino IDE does not currently meet the required standards for the package manager:

https://github.com/arduino/Arduino/pull/2703