You can look at JavaBrowser's preferences to see what's in the classpath, which directories it's using for documentation, and which directories it's using for source code. I don't think Sun's source code or documentation is on the system by default.
I'm sure you could download the Sun source and/or docs and install them on your system and add the paths to JavaBrowser.
As a sort of alternative, you could also install the jad decompiler and ReSource (search on
Softrak). ReSource will use jad to decompile the selected class and show it to you. It adds another button along the bottom in JavaBrowser. It's a little flaky but it does work.