Please refer to the project page for more information:
http://code.google.com/p/zxing/
in particular:
http://code.google.com/p/zxing/wiki/GettingStarted

To get started, you can try building and running the command-line client;
you will need to have Apache's Ant tool installed to run this:

ant -f core/build.xml
ant -f javase/build.xml
java -cp javase/javase.jar:core/core.jar com.google.zxing.client.j2se.CommandLineRunner [URL]