In your project's properties, go to Java Build Path, and on the Libraries tab, expand the JAR files - there will be source files entry - select that and click the Edit button and point it to the source code.
Where to get the source depends on the JAR; usually you can get the source form the same location from which you obtained the binary JAR. If you are using
maven, you can ask it to also get the source JAR (which is made extremely easy if you have the m2eclispe plugin) and then access the source in your local repository.
