To use the CellNoise library and its noise function, you have to download the package cellnoise.zip.
Unpack cellnoise.zip into your Processing sketches folder's libraries folder. Like: ./Processing/libraries/cellnoise/library/cellnoise.jar
Detailed instructions at www.processing.org
Create a folder named libraries/cellnoise/library in your Processing installation and copy the file ./library/cellnoise.jar of the zip file into it.
Here is an example on how it can look like: