How to import a library to Processing?

In order to be able to use a library written for Processing you have to make it visible for Processing. Probably you have .jar files downloaded. To do that, copy the downloaded .jar files to a folder in your sketch's directory called code. You could also use the Sketch > Import Library menu in the program.

In the case of sunflow4p5 you need to import sunflow.jar, janino.jar and sunflow-wrapper-.jar

Source: http://processing.org/reference/libraries/