Use version 5 at https://java.net/projects/javacc/downloads . Version 6.0 is missing the scripts to run JavaCC. If you really want to use version 6.0 you can find the appropriate scripts in version 5, just copy them to version 6's bin directory and away you go.
Update (2020): Since version 6 is now harder to find, I have put a copy at www.engr.mun.ca/~theo/JavaCC/javacc-6.1.0.zip