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
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 > Import Library > Add Library > Select the zip file.
Full library instructions are here.
Include a Library
#include < Servo.h >