You add it like a regular library.
If in Eclipse, add it via the Project Properties.
If in the PDE, either drag the joda-time-2.0.jar on the PDE window (will create a code folder with the jar), or put it in the libraries folder of your sketchbook, following the usual rules / path. Meaning you probably need to rename this file as jodatime.jar, for example (so that the folder has the name simple name than the jar).

quite easy, thanks