You need the following tools:
You can manually checkout sources from the repository by using the command line:
hg clone https://kenai.com/hg/jrawio~src
NetBeans 6.7.1 has a direct menu to check out projects out of Kenai that automates the whole process. Please refer to the Development section for more information, in particular about how source code is managed.
Alternatively you can browse the source repository.