After at least Arduino1.0.1 the LiquidCrystal_I2C library is included.
If you do download one be sure to use this https://bitbucket.org/fmalpartida/new-liquidcrystal/downloads
. It is the same one used by the official release and is faster and more complete than any other. Thanks go to FM for his outstanding work. There are complete instructions included in the .zip. If you do put it in the Arduino sketchbook /librariesfolder (recomended) makes sure you find the actual Arduino program folder, go to the libraries folder there and move the old LiquidCrystal library to a completely different location or delete it entirely. Two libraries with the same name will cause conflicts.