You will have to locate the desired part library on-line ( partname.fzpz ) and download it to your PC. Then in Fritzing, select “Mine” My Parts, you can then use the menu icon in the top right or right click on a blank area in the My Parts palette. Select import, browse for the part (where you downloaded it) on your PC, select it and click open to install the part. It should then be available, but make sure you also choose to save the part and your bin when you exit Fritzing.
I have a 20x4 LCD-I2C part in this forum ( LCD2004-I2C.fzpz ) pending approval for inclusion, the link to the original 16x2 LCD-I2C ( LCD1602-I2C.fzpz ) and the link to my part are below:
Here is the link to the 16x2 LCD-I2C part:
http://forum.fritzing.org/uploads/default/original/2X/7/70f90addd7883759e4a0d06a934946c2be8aa6c1.fzpz
Here is the link to my modified 20x4 LCD-I2C part:
http://forum.fritzing.org/uploads/default/original/2X/3/396ac45a9caa834358c9439a959ef2a6203c7a2d.fzpz