Go to https://github.com/carlosdelfino/DuinOS/Downloads and get the last version.

0. Close the Arduino IDE (all instances).
1. Copy the arduino.DuinOS.* folder to the \hardware\arduino*\cores folder.
2. Rename your current \hardware\arduino*\boards.txt file.
3. Copy the \boards.txt deployed with DuinOS to the \hardware\arduino* folder, remember eatch borads..txt for respective hardware folder.
4. Rename your current \lib\keywords.txt file.
5. Copy the keywords.txt deployed with DuinOS to the \lib folder.
6. Start the Arduino IDE.
7. Please select any DuinOS board from the Tools->Board menu, like the "Arduino Duemilanove or Nano w/ ATmega328 + DuinOS".
8. Now, you can compile any standard project, but we suggest the \examples\DuinOS\MoreComplexBlinking deployed with DuinOS.