Adding A Library Using the IDE Library Manager

Open the Arduino library manager:  Menu > Sketch > Include Library > Manage Libraries

Adding A Library For All Projects

On Windows

Copy the library directory into the folder: My Documents /Arduino/libraries/

On A Mac

Copy the library directory into the folder: ~/Documents/Arduino/libraries/

Restart the Arduino IDE and the library will be available to add from Menu > Sketch > Import Library

Installing A Library

To install a library which is not part of the pre-installed libraries get the zip file of the library.

Menu > Sketch > Include Library > Add .zip Library > Select the zip file.

Full library instructions are here.

Include a Library

#include < Servo.h >